fix warning/error: compound assignment with 'volatile'-qualified left operand is deprecated #925

Job	Run time
tock-build	1m 10s
esp-build	2m 56s
rpi-build	52s
arduino:avr:uno	4m 14s
arduino:avr:mega	7s
arduino:mbed:nano33ble	5s
arduino:mbed:envie_m4	7s
arduino:megaavr:uno2018	9s
arduino:sam:arduino_due_x	5s
arduino:samd:arduino_zero_native	7s
adafruit:samd:adafruit_feather_m0	8s
adafruit:nrf52:feather52832	4s
esp32:esp32:esp32	6s
esp8266:esp8266:generic	7s
Intel:arc32:arduino_101	7s
SparkFun:apollo3:sfe_artemis	6s
STMicroelectronics:stm32:GenF3:pnum=BLACKPILL_F303CC	6s
STMicroelectronics:stm32:Nucleo_64:pnum=NUCLEO_WL55JC1	5s
stm32duino:STM32F1:mapleMini	5s
MegaCoreX:megaavr:4809	7s
arduino:mbed_rp2040:pico	4s
rp2040:rp2040:rpipico	4s
CubeCell:CubeCell:CubeCell-Board	6s
MegaCore:avr:1281	8s
teensy:avr:teensy41	5s
arduino:renesas_uno:minima	6s
rpi-test	7s
	11m 33s

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