[LoRaWAN] Properly size fopts buffer #949

Job	Run time
arduino:avr:uno	4m 19s
arduino:avr:mega	7s
arduino:mbed:nano33ble	5s
arduino:mbed:envie_m4	6s
arduino:megaavr:uno2018	9s
arduino:sam:arduino_due_x	7s
arduino:samd:arduino_zero_native	6s
adafruit:samd:adafruit_feather_m0	6s
adafruit:nrf52:feather52832	7s
esp-build	2m 49s
esp32:esp32:esp32	7s
esp8266:esp8266:generic	8s
Intel:arc32:arduino_101	5s
rpi-build	52s
SparkFun:apollo3:sfe_artemis	6s
STMicroelectronics:stm32:GenF3:pnum=BLACKPILL_F303CC	8s
tock-build	1m 27s
STMicroelectronics:stm32:Nucleo_64:pnum=NUCLEO_WL55JC1	7s
stm32duino:STM32F1:mapleMini	4s
MegaCoreX:megaavr:4809	4s
arduino:mbed_rp2040:pico	6s
rp2040:rp2040:rpipico	5s
CubeCell:CubeCell:CubeCell-Board	7s
MegaCore:avr:1281	7s
arduino:renesas_uno:minima	5s
rpi-test	11s
	11m 50s

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