AP_NavEKF3: Don't allow mag to interfere with use of external yaw sensor

libraries/AP_NavEKF3/AP_NavEKF3_Control.cpp

@@ -450,7 +450,7 @@ bool NavEKF3_core::readyToUseRangeBeacon(void) const
 // return true if we should use the compass
 bool NavEKF3_core::use_compass(void) const
 {
-    return _ahrs->get_compass() && _ahrs->get_compass()->use_for_yaw(magSelectIndex) && !allMagSensorsFailed;
+    return (frontend->_magCal != 5) && _ahrs->get_compass() && _ahrs->get_compass()->use_for_yaw(magSelectIndex) && !allMagSensorsFailed;
 }
 
 /*

libraries/AP_NavEKF3/AP_NavEKF3_MagFusion.cpp

@@ -261,7 +261,7 @@ void NavEKF3_core::SelectMagFusion()
     magFusePerformed = false;
 
     // Handle case where we are using an external yaw sensor instead of a magnetomer
-    if ((frontend->_magCal == 5) && !use_compass()) {
+    if ((frontend->_magCal == 5)) {
         if (storedYawAng.recall(yawAngDataDelayed,imuDataDelayed.time_ms)) {
             if (tiltAlignComplete && !yawAlignComplete) {
                 alignYawAngle();