AP_ExternalAHRS: Correct the judgment #33183

Job	Run time
build (chibios, Durandal)	19m 39s
build (chibios, MatekF405)	17m 54s
build (chibios, KakuteF7)	15m 52s
build (chibios, MatekH743-bdshot)	20m 8s
build (chibios, Pixhawk1-1M)	16m 57s
build (chibios, MatekF405-CAN)	4m 27s
build (chibios, DrotekP3Pro)	4m 27s
build (chibios, Hitec-Airspeed)	5m 19s
build (chibios, f103-GPS)	5m 29s
build (disco, armhf)	16m 4s
	2h 6m 16s

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