AP_NavEKF: Fix bug in bias rate of change limiting

This doesn't change the behaviour, but means the result is achieved using the correct parameters, and comments are consistent
AP_NavEKF: Add public method reporting horizontal speed limit

This is required if using optical flow, as depending on height, the speed must be limited to prevent the sensor saturating
AP_NavEKF: Apply single definition of using optical flow
AP_NavEKF: Improve comments in setInhibitGPS public method