setup the MODESWITCH channel as our OVERRIDE_CHAN, allowing for
instant manual passthru in px4io when OVERRIDE_CHAN goes above 1750
This makes for faster switching, and avoids bugs where a still enabled
FMU channel disables override
When arming we need to ensure that we don't enable any channels
otherwise PX4IO will think that FMU is active and will stop running
the internal RC mixer