BUG FIX - Fixes 3d flight mixer so that it uses midrc

Previously it was using a hardcoded value of 1500.

Without this commit it would mean that any midrc configuration was not
being applied to 3d flight.

lready applied in a6c22d21159198b7ea566bc83f9b55d127e0f318
Conflicts:

	src/flight_mixer.c