AP_BLHeli: normalize ESC index correctly with iomcu #45360

Job	Run time
build (stm32f7, chibios, 10)	3m 46s
build (stm32h7, chibios, 10)	3m 17s
build (fmuv2-plane, chibios, 10)	1m 49s
build (periph-build, chibios, 10)	3m 40s
build (iofirmware, chibios, 10)	1m 50s
build (CubeOrange-bootloader, chibios, 10)	51s
build (fmuv3-bootloader, chibios, 10)	49s
build (revo-bootloader, chibios, 10)	49s
build (stm32h7-debug, chibios, 10)	2m 10s
build (fmuv3, chibios, 10)	5m 39s
build (revo-mini, chibios, 10)	4m 30s
build (MatekF405-Wing, chibios, 10)	4m 39s
build (CubeOrange-ODID, chibios, 10)	2m 36s
build (CubeRedPrimary-bootloader, chibios, 10)	56s
build (configure-all, chibios, 10)	11m 16s
build (build-options-defaults-test, chibios, 10)	6m 52s
build (signing, chibios, 10)	2m 38s
build (CubeOrange-PPP, chibios, 10)	2m 1s
build (CubeRed-EKF2, chibios, 10)	1m 59s
build (SOHW, chibios, 10)	3m 41s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 13s
build (new-check, chibios, 10)	4m 17s
	1h 11m 18s

Provide feedback