AP_MotorsHeli_RSC: Make variables in constructor const #347

Job	Run time
build (stm32f7, chibios, 10)	7m 59s
build (stm32h7, chibios, 10)	7m 4s
build (fmuv2-plane, chibios, 10)	4m 52s
build (periph-build, chibios, 10)	5m 31s
build (iofirmware, chibios, 10)	3m 49s
build (CubeOrange-bootloader, chibios, 10)	3m 1s
build (fmuv3-bootloader, chibios, 10)	2m 58s
build (revo-bootloader, chibios, 10)	3m 13s
build (stm32h7-debug, chibios, 10)	5m 45s
build (fmuv3, chibios, 10)	10m 32s
build (revo-mini, chibios, 10)	8m 31s
build (MatekF405-Wing, chibios, 10)	9m 9s
build (CubeOrange-ODID, chibios, 10)	6m 25s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 28s
build (configure-all, chibios, 10)	12m 37s
build (build-options-defaults-test, chibios, 10)	13m 51s
build (signing, chibios, 10)	5m 42s
build (CubeOrange-PPP, chibios, 10)	5m 14s
build (SOHW, chibios, 10)	7m 11s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 24s
build (new-check, chibios, 10)	9m 22s
	2h 19m 38s

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