Fix invalid reference casting compile error in spi.rs

NOTE: I have not tested this on hardware, just added the allow attribute.

Also see [this](rust-lang/rust#116410) issue

Author: JanekGraff

src/spi.rs

@@ -413,7 +413,10 @@ where
 
     fn send_u8(&mut self, byte: u8) {
         // NOTE(write_volatile) see note above
-        unsafe { ptr::write_volatile(&self.spi.dr as *const _ as *mut u8, byte) }
+        #[allow(invalid_reference_casting)]
+        unsafe {
+            ptr::write_volatile(&self.spi.dr as *const _ as *mut u8, byte)
+        }
     }
 
     fn read_u16(&mut self) -> u16 {
@@ -423,7 +426,10 @@ where
 
     fn send_u16(&mut self, byte: u16) {
         // NOTE(write_volatile) see note above
-        unsafe { ptr::write_volatile(&self.spi.dr as *const _ as *mut u16, byte) }
+        #[allow(invalid_reference_casting)]
+        unsafe {
+            ptr::write_volatile(&self.spi.dr as *const _ as *mut u16, byte)
+        }
     }
 
     pub fn release(self) -> (SPI, (SCKPIN, MISOPIN, MOSIPIN)) {