Change "ctrl_length" variable name to "ctrl_data" for control writes

This is indeed the right data type, the length being read from the
actual bytearray data. This misunderstanding led to #10.

ratctl.py

@@ -51,6 +51,7 @@ def __init__(self):
         self.ctrl_value = 0
         self.ctrl_index = 0
         self.ctrl_length = 0
+        self.ctrl_data = b''
         self.currentDevice = {}
         self.initiate()
 
@@ -154,7 +155,7 @@ def sendDpi(self, dpi):
         self.ctrl_request_type = 0x40
         self.ctrl_request = 145
         self.ctrl_index = 115
-        self.ctrl_length = b''
+        self.ctrl_data = b''
         for mode in range(0, 4):
             for axis in range(0, 2):
                 self.ctrl_value = 0x1000 * (mode + 1)
@@ -174,15 +175,15 @@ def sendMode(self, Mode):
         self.ctrl_request_type = 0x40
         self.ctrl_request = 145
         self.ctrl_index = 116
-        self.ctrl_length = b''
+        self.ctrl_data = b''
         self.ctrl_value = 0x1000 * (Mode + 1)
         self.controlWrite()
 
     def resetDpi(self):
         self.ctrl_request_type = 0x40
         self.ctrl_request = 145
         self.ctrl_index = 115
-        self.ctrl_length = b''
+        self.ctrl_data = b''
         self.ctrl_value = 0x0000
         # Reset packet.
         self.controlWrite()
@@ -208,7 +209,7 @@ def controlWrite(self):
             self.handle.controlWrite(self.ctrl_request_type,
                                      self.ctrl_request,
                                      self.ctrl_value, self.ctrl_index,
-                                     self.ctrl_length)
+                                     self.ctrl_data)
         except:
             self.hasHandle = 0