[LoRaWAN] Move TX power logic to function #889

Job	Run time
esp-build	2m 50s
rpi-build	52s
arduino:avr:uno	4m 13s
tock-build	1m 52s
arduino:avr:mega	5s
arduino:mbed:nano33ble	4s
arduino:mbed:envie_m4	5s
arduino:megaavr:uno2018	7s
arduino:sam:arduino_due_x	7s
arduino:samd:arduino_zero_native	7s
adafruit:samd:adafruit_feather_m0	5s
adafruit:nrf52:feather52832	5s
esp32:esp32:esp32	8s
esp8266:esp8266:generic	7s
Intel:arc32:arduino_101	5s
SparkFun:apollo3:sfe_artemis	5s
STMicroelectronics:stm32:GenF3:pnum=BLACKPILL_F303CC	7s
STMicroelectronics:stm32:Nucleo_64:pnum=NUCLEO_WL55JC1	7s
stm32duino:STM32F1:mapleMini	7s
MegaCoreX:megaavr:4809	7s
arduino:mbed_rp2040:pico	5s
rp2040:rp2040:rpipico	5s
CubeCell:CubeCell:CubeCell-Board	6s
MegaCore:avr:1281	5s
teensy:avr:teensy41	9s
arduino:renesas_uno:minima	7s
rpi-test	7s
	12m 9s

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