AP_DroneCAN: correct narrowing-conversion errors #3647

Job	Run time
build (stm32f7, chibios, 10)	10m 9s
build (stm32h7, chibios, 10)	9m 12s
build (fmuv2-plane, chibios, 10)	4m 9s
build (periph-build, chibios, 10)	6m 26s
build (iofirmware, chibios, 10)	1m 37s
build (CubeOrange-bootloader, chibios, 10)	49s
build (fmuv3-bootloader, chibios, 10)	48s
build (revo-bootloader, chibios, 10)	52s
build (stm32h7-debug, chibios, 10)	5m 27s
build (fmuv3, chibios, 10)	12m 26s
build (revo-mini, chibios, 10)	8m 34s
build (MatekF405-Wing, chibios, 10)	9m 15s
build (CubeOrange-ODID, chibios, 10)	6m 38s
build (CubeRedPrimary-bootloader, chibios, 10)	50s
build (configure-all, chibios, 10)	10m 41s
build (build-options-defaults-test, chibios, 10)	19m 53s
build (signing, chibios, 10)	6m 33s
build (CubeOrange-PPP, chibios, 10)	5m 24s
build (CubeRed-EKF2, chibios, 10)	5m 10s
build (SOHW, chibios, 10)	9m 5s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 29s
build (new-check, chibios, 10)	10m 57s
	2h 26m 24s

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