Use core::ptr::addr_of! in spi.rs instead of suppressing the compile error

Author: JanekGraff

src/spi.rs

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