AP_Periph: fixup FSO fans #1214

Job	Run time
build (stm32f7, chibios, 10)	2m 2s
build (stm32h7, chibios, 10)	1m 34s
build (fmuv2-plane, chibios, 10)	2m 10s
build (periph-build, chibios, 10)	2m 11s
build (iofirmware, chibios, 10)	1m 24s
build (CubeOrange-bootloader, chibios, 10)	57s
build (fmuv3-bootloader, chibios, 10)	52s
build (revo-bootloader, chibios, 10)	1m 20s
build (stm32h7-debug, chibios, 10)	31s
build (fmuv3, chibios, 10)	3m 46s
build (revo-mini, chibios, 10)	2m 4s
build (MatekF405-Wing, chibios, 10)	3m 4s
build (CubeOrange-ODID, chibios, 10)	2m 31s
build (CubeRedPrimary-bootloader, chibios, 10)	1m 8s
build (configure-all, chibios, 10)	4m 14s
build (build-options-defaults-test, chibios, 10)	4m 17s
build (signing, chibios, 10)	2m 5s
build (CubeOrange-PPP, chibios, 10)	2m 0s
build (CubeOrange-SOHW, chibios, 10)	2m 5s
build (Pixhawk6X-PPPGW, chibios, 10)	1m 35s
	41m 50s

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