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

Marlin/src/HAL/STM32F1/HAL_SPI.cpp

  * @details
  */
 uint8_t spiRec() {
-  uint8_t returnByte = SPI.transfer(ff);
+  uint8_t returnByte = SPI.transfer(0xFF);
   return returnByte;
 }
 
 #if ENABLED(SPI_EEPROM)
 
 // Read single byte from specified SPI channel
-uint8_t spiRec(uint32_t chan) { return SPI.transfer(ff); }
+uint8_t spiRec(uint32_t chan) { return SPI.transfer(0xFF); }
 
 // Write single byte to specified SPI channel
 void spiSend(uint32_t chan, byte b) { SPI.send(b); }

Marlin/src/libs/W25Qxx.cpp

  * @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;
 }