AP_NavEKF2: Speed improve switch-over to backup magnetometer

When switching over to a back up magnetometer, ensure that the earth field estimate are reset. other wise mag earth field estimates due to the previous failed mag could cause data from the new mag to be rejected.

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -185,7 +185,13 @@ void NavEKF2_core::readMagData()
                     stateStruct.body_magfield.zero();
                     // clear the measurement buffer
                     storedMag.reset();
-                    }
+                    // clear the data waiting flag so that we do not use any data pending from the previous sensor
+                    magDataToFuse = false;
+                    // request a reset of the magnetic field states
+                    magStateResetRequest = true;
+                    // declare the field unlearned so that the reset request will be obeyed
+                    magFieldLearned = false;
+                }
             }
         }