AR_Motors: fix prearm for omni outputs #93

Job	Run time
build (stm32f7, chibios, 10)	5m 25s
build (stm32h7, chibios, 10)	3m 33s
build (fmuv2-plane, chibios, 10)	1m 53s
build (periph-build, chibios, 10)	4m 58s
build (iofirmware, chibios, 10)	1m 46s
build (CubeOrange-bootloader, chibios, 10)	56s
build (fmuv3-bootloader, chibios, 10)	57s
build (revo-bootloader, chibios, 10)	53s
build (stm32h7-debug, chibios, 10)	2m 9s
build (fmuv3, chibios, 10)	5m 20s
build (revo-mini, chibios, 10)	4m 8s
build (MatekF405-Wing, chibios, 10)	4m 47s
build (CubeOrange-ODID, chibios, 10)	2m 28s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 13s
build (configure-all, chibios, 10)	9m 2s
build (build-options-defaults-test, chibios, 10)	6m 9s
build (signing, chibios, 10)	2m 38s
build (CubeOrange-PPP, chibios, 10)	2m 3s
build (CubeOrange-SOHW, chibios, 10)	2m 6s
	1h 2m 24s

Provide feedback