mysql_grant: Fix the read of grant option.

If multiple `mysql_grant` are defined for the same user,
the grant value of the last one will be set for all of them.

The example (that I used in the acceptance tests) to reproduce this bug:

```hcl
terraform {
  required_providers {
    mysql = {
      source = "winebarrel/mysql"
    }
  }
}

resource "mysql_database" "db1" {
  name = "db1"
}

resource "mysql_database" "db2" {
  name = "db2"
}

resource "mysql_user" "test" {
  user = "test"
  host = "%"
}

resource "mysql_grant" "test_db1" {
  user       = mysql_user.test.user
  host       = mysql_user.test.host
  database   = mysql_database.db1.name
  privileges = ["SELECT"]
}

resource "mysql_grant" "test_db2" {
  user       = mysql_user.test.user
  host       = mysql_user.test.host
  database   = mysql_database.db2.name
  privileges = ["SELECT"]
  grant      = true
}
```

After the first apply of these resources, there will be a dirty plan:

```bash
●examples❯ terraform apply -auto-approve
[...]
mysql_database.db2: Creating...
mysql_user.test: Creating...
mysql_database.db1: Creating...
mysql_user.test: Creation complete after 0s [id=test@%]
mysql_database.db1: Creation complete after 0s [id=db1]
mysql_database.db2: Creation complete after 0s [id=db2]
mysql_grant.test_db1: Creating...
mysql_grant.test_db2: Creating...
mysql_grant.test_db2: Creation complete after 0s [id=test@%:`db2`]
mysql_grant.test_db1: Creation complete after 0s [id=test@%:`db1`]

Apply complete! Resources: 5 added, 0 changed, 0 destroyed.

●examples❯ terraform plan
mysql_user.test: Refreshing state... [id=test@%]
mysql_database.db1: Refreshing state... [id=db1]
mysql_database.db2: Refreshing state... [id=db2]
mysql_grant.test_db2: Refreshing state... [id=test@%:`db2`]
mysql_grant.test_db1: Refreshing state... [id=test@%:`db1`]

Terraform used the selected providers to generate the following execution plan. Resource actions are indicated with the following symbols:
-/+ destroy and then create replacement

Terraform will perform the following actions:

  # mysql_grant.test_db1 must be replaced
-/+ resource "mysql_grant" "test_db1" {
      ~ grant      = true -> false # forces replacement
      ~ id         = "test@%:`db1`" -> (known after apply)
        # (6 unchanged attributes hidden)
    }

Plan: 1 to add, 0 to change, 1 to destroy.
```

mysql/resource_grant.go

@@ -268,10 +268,10 @@ func ReadGrant(d *schema.ResourceData, meta interface{}) error {
 	for _, grant := range grants {
 		if grant.Database == database && grant.Table == table {
 			privileges = grant.Privileges
-		}
-
-		if grant.Grant {
-			grantOption = true
+			if grant.Grant {
+				grantOption = true
+			}
+			break
 		}
 	}
 

mysql/resource_grant_test.go

@@ -46,6 +46,67 @@ func TestAccGrant(t *testing.T) {
 	})
 }
 
+func TestAccGrant_grantOption(t *testing.T) {
+	randInt := rand.Intn(100)
+	db1 := fmt.Sprintf("tf-test-%d", randInt)
+	db2 := fmt.Sprintf("tf-test-%d", randInt+1)
+
+	config := fmt.Sprintf(`
+resource "mysql_database" "db1" {
+  name = "%s"
+}
+
+resource "mysql_database" "db2" {
+  name = "%s"
+}
+
+resource "mysql_user" "test" {
+  user     = "jdoe-%d"
+  host     = "example.com"
+}
+
+resource "mysql_grant" "test_db1" {
+  user       = mysql_user.test.user
+  host       = mysql_user.test.host
+  database   = mysql_database.db1.name
+  privileges = ["SELECT"]
+}
+
+resource "mysql_grant" "test_db2" {
+  user       = mysql_user.test.user
+  host       = mysql_user.test.host
+  database   = mysql_database.db2.name
+  privileges = ["SELECT"]
+  grant = true
+}
+`, db1, db2, randInt)
+
+	resource.Test(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccGrantCheckDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: config,
+				Check: resource.ComposeTestCheckFunc(
+					testAccPrivilegeExists("mysql_grant.test_db1", "SELECT"),
+					resource.TestCheckResourceAttr("mysql_grant.test_db1", "user", fmt.Sprintf("jdoe-%d", randInt)),
+					resource.TestCheckResourceAttr("mysql_grant.test_db1", "host", "example.com"),
+					resource.TestCheckResourceAttr("mysql_grant.test_db1", "database", db1),
+					resource.TestCheckResourceAttr("mysql_grant.test_db1", "table", "*"),
+					resource.TestCheckResourceAttr("mysql_grant.test_db1", "grant", "false"),
+
+					testAccPrivilegeExists("mysql_grant.test_db2", "SELECT"),
+					resource.TestCheckResourceAttr("mysql_grant.test_db2", "user", fmt.Sprintf("jdoe-%d", randInt)),
+					resource.TestCheckResourceAttr("mysql_grant.test_db2", "host", "example.com"),
+					resource.TestCheckResourceAttr("mysql_grant.test_db2", "database", db2),
+					resource.TestCheckResourceAttr("mysql_grant.test_db2", "grant", "true"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccGrant_role(t *testing.T) {
 	rand.Seed(time.Now().UnixNano())
 	dbName := fmt.Sprintf("tf-test-%d", rand.Intn(100))