Silent a Rust compiler warning on invalid reference casting

Using such a cast to set the `dr` register inside an `unsafe` block is
an acceptable way to access the data.

src/spi.rs

@@ -353,6 +353,7 @@ where
         unsafe { ptr::read_volatile(&self.spi.dr as *const _ as *const FrameSize) }
     }
 
+    #[allow(invalid_reference_casting)]
     fn write_data_reg(&mut self, data: FrameSize) {
         // NOTE(write_volatile) see note above
         unsafe { ptr::write_volatile(&self.spi.dr as *const _ as *mut FrameSize, data) }