Use 0xFF (not 'ff') for byte transfer

Marlin/src/libs/W25Qxx.cpp

@@ -80,16 +80,14 @@ void W25QXXFlash::init(uint8_t spiRate) {
  * @brief  Receive a single byte from the SPI port.
  *
  * @return Byte received
- *
- * @details
  */
 uint8_t W25QXXFlash::spi_flash_Rec() {
-  uint8_t returnByte = SPI.transfer(ff);
+  const uint8_t returnByte = SPI.transfer(0xFF);
   return returnByte;
 }
 
 uint8_t W25QXXFlash::spi_flash_read_write_byte(uint8_t data) {
-  uint8_t returnByte = SPI.transfer(data);
+  const uint8_t returnByte = SPI.transfer(data);
   return returnByte;
 }