EKF: consolidate covariance corrections

Combines the forced symmetry, variance limiting and zeroing of covariances for unwanted states in the one function.
This ensures a consistent correction is applied after every covariance prediction or correction.

EKF/airspeed_fusion.cpp

@@ -187,8 +187,7 @@ void Ekf::fuseAirspeed()
 			}
 
 			// correct the covariance marix for gross errors
-			makeSymmetrical();
-			limitCov();
+			fixCovarianceErrors();
 
 			// apply the state corrections
 			fuse(Kfusion, _airspeed_innov);