Plane: fixup throttle trim on failsafe
Plane: treat PWM below 900 as invalid for mode switch

the FrSky X8R receiver will briefly give a PWM of 874 on channel 5
when connection is lost to the transmitter. We need to ensure this
doesn't cause a mode switch
Rover: fix for X8R receiver on SBUS in failsafe

the X8R briefly gives PWM of 874 on all channels when the receiver is
setup as "no pulses" failsafe handling. This was interpreted as switch
position 0, which was AUTO for me.

This change rejects pulse widths below 900 for mode changes