AP_NavEKF3: 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_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -292,6 +292,7 @@ void NavEKF3_core::readMagData()
                     magStateResetRequest = true;
                     // declare the field unlearned so that the reset request will be obeyed
                     magFieldLearned = false;
+                    break;
                 }
             }
         }