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.
Merge pull request #93 from hydra/3d-flight-midrc-fix

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