gps: fixed filter initialization bug around gps

- do not gps reference altitude to zero in case gps checks pass before the
filter initialized
- reset the filtered gps position and position derivative filters in case
we are in air or there is movement on the ground

Signed-off-by: RomanBapst <bapstroman@gmail.com>

EKF/gps_checks.cpp

@@ -194,10 +194,13 @@ bool Ekf::gps_is_good(const gps_message &gps)
 		_gps_check_fail_status.flags.hspeed = false;
 		_gps_drift_updated = false;
 
+		resetGpsDriftCheckFilters();
+
 	} else {
 		// This is the case where the vehicle is on ground and IMU movement is blocking the drift calculation
 		_gps_drift_updated = true;
 
+		resetGpsDriftCheckFilters();
 	}
 
 	// save GPS fix for next time