EKF: Add missing optical flow ground motion protection
EKF: Add missing optical flow ground motion protection

Motion compensated optical flow rates are supposed to be zeroed if reported flow quality is below the minimum threshold value when on ground.
The comments and logic have been amended to be consistent and make the design intent clearer.

PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl