Make enums into implicit char

Marlin/src/HAL/HAL_DUE/usb/genclk.h

Marlin/src/HAL/HAL_DUE/usb/pll.h

Marlin/src/HAL/HAL_DUE/usb/sbc_protocol.h

Marlin/src/HAL/HAL_DUE/usb/spc_protocol.h

Marlin/src/HAL/HAL_DUE/usb/usb_protocol.h

Marlin/src/HAL/HAL_DUE/usb/usb_protocol_cdc.h

Marlin/src/HAL/HAL_DUE/usb/usb_protocol_msc.h

Marlin/src/HAL/HAL_STM32F1/HAL_Stm32f1.cpp

Marlin/src/core/enum.h

Marlin/src/core/serial.h

Marlin/src/feature/bedlevel/mbl/mesh_bed_leveling.h

Marlin/src/feature/bedlevel/ubl/ubl.h

Marlin/src/feature/pause.h

Marlin/src/feature/tmc_util.cpp

Marlin/src/feature/tmc_util.h

Marlin/src/gcode/calibrate/G33.cpp

Marlin/src/gcode/gcode.h

Marlin/src/libs/stopwatch.h

Marlin/src/module/endstops.h

Marlin/src/module/motion.h