AP_NavEKF2: fixed EKF compass switching

when we had 3 compasses the lack of the 'break' meant when we switched
compass in flight we would always switch back instantly to the one
that we had just rejected.

libraries/AP_NavEKF2/AP_NavEKF2_Measurements.cpp

@@ -234,6 +234,7 @@ void NavEKF2_core::readMagData()
                     magStateResetRequest = true;
                     // declare the field unlearned so that the reset request will be obeyed
                     magFieldLearned = false;
+                    break;
                 }
             }
         }