Plane: fix reversing with steering assist #1321

Job	Run time
build-gcc-heli	3m 24s
build (base)	3m 11s
build (clang)	3m 23s
autotest (sitltest-copter-tests1a)	11m 41s
autotest (sitltest-copter-tests1b)	12m 5s
autotest (sitltest-copter-tests1c)	14m 7s
autotest (sitltest-copter-tests1d)	14m 0s
autotest (sitltest-copter-tests1e)	13m 43s
autotest (sitltest-copter-tests2a)	11m 11s
autotest (sitltest-copter-tests2b)	34m 45s
autotest-heli (sitltest-heli)	7m 14s
	2h 8m 44s

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