SITL: fix json airspeed #37960

Job	Run time
build (base)	4m 17s
build-gcc-heli	4m 16s
build (clang)	4m 10s
autotest-heli (sitltest-heli)	10m 31s
autotest (sitltest-copter-tests1a)	7m 16s
autotest (sitltest-copter-tests1b)	9m 25s
autotest (sitltest-copter-tests1c)	10m 50s
autotest (sitltest-copter-tests1d)	7m 52s
autotest (sitltest-copter-tests1e)	8m 20s
autotest (sitltest-copter-tests2a)	9m 2s
autotest (sitltest-copter-tests2b)	37m 23s
	1h 53m 22s

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