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/ekf.h

@@ -893,4 +893,5 @@ private:
 	// Returns true if the reset was successful
 	bool resetYawToEKFGSF();
 
+	void resetGpsDriftCheckFilters();
 };