AP_DroneCAN: Fix constructor initialization order #893

Job	Run time
build (stm32f7, chibios, 10)	10m 21s
build (stm32h7, chibios, 10)	10m 3s
build (fmuv2-plane, chibios, 10)	6m 38s
build (periph-build, chibios, 10)	9m 51s
build (iofirmware, chibios, 10)	4m 1s
build (CubeOrange-bootloader, chibios, 10)	3m 18s
build (fmuv3-bootloader, chibios, 10)	3m 3s
build (revo-bootloader, chibios, 10)	3m 2s
build (stm32h7-debug, chibios, 10)	7m 27s
build (fmuv3, chibios, 10)	17m 19s
build (revo-mini, chibios, 10)	8m 28s
build (MatekF405-Wing, chibios, 10)	9m 18s
build (CubeOrange-ODID, chibios, 10)	9m 9s
build (CubeRedPrimary-bootloader, chibios, 10)	3m 3s
build (configure-all, chibios, 10)	12m 28s
build (build-options-defaults-test, chibios, 10)	11m 14s
build (signing, chibios, 10)	8m 2s
build (CubeOrange-PPP, chibios, 10)	7m 31s
build (CubeOrange-SOHW, chibios, 10)	6m 56s
build (Pixhawk6X-PPPGW, chibios, 10)	3m 49s
	2h 35m 1s

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