Fix compiler warnings: sketch/Marlin_main.cpp: In function 'void setup()':

sketch/Marlin_main.cpp:1478:57: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if ((farm_mode == 0xFF && farm_no == 0) || (farm_no == 0xFFFF)) farm_mode = false; //if farm_mode has not been stored to eeprom yet and farm number is set to zero or EEPROM is fresh, deactivate farm mode
                                                         ^
sketch/Marlin_main.cpp:1479:17: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  if (farm_no == 0xFFFF) farm_no = 0;
                 ^

Firmware/Marlin_main.cpp

@@ -1475,8 +1475,8 @@ void setup()
 #endif
 	farm_mode = eeprom_read_byte((uint8_t*)EEPROM_FARM_MODE);
 	EEPROM_read_B(EEPROM_FARM_NUMBER, &farm_no);
-	if ((farm_mode == 0xFF && farm_no == 0) || (farm_no == 0xFFFF)) farm_mode = false; //if farm_mode has not been stored to eeprom yet and farm number is set to zero or EEPROM is fresh, deactivate farm mode 
-	if (farm_no == 0xFFFF) farm_no = 0;
+	if ((farm_mode == 0xFF && farm_no == 0) || (farm_no == static_cast<int>(0xFFFF))) farm_mode = false; //if farm_mode has not been stored to eeprom yet and farm number is set to zero or EEPROM is fresh, deactivate farm mode
+	if (farm_no == static_cast<int>(0xFFFF)) farm_no = 0;
 	if (farm_mode)
 	{
 		prusa_statistics(8);