mc mixer: additional safe limiting of mixed out

Fixes error on FMUv1 (cutting outs at full throttle saturation) caused by this lines:
https://github.com/PX4/Firmware/blob/master/src/drivers/px4fmu/fmu.cpp#L652

CRITICAL FIX
Merge ASAP after testing.
Not tested yet, should be tested on FMUv1 and v2 (at least bench) at full thrust.