Copter: Stop autotune repeatedly saving gains

ArduCopter/control_autotune.pde

@@ -984,6 +984,8 @@ static void autotune_save_tuning_gains()
         // update GCS and log save gains event
         autotune_update_gcs(AUTOTUNE_MESSAGE_SAVED_GAINS);
         Log_Write_Event(DATA_AUTOTUNE_SAVEDGAINS);
+        // reset Autotune so that gains are not saved again and autotune can be run again.
+        autotune_state.mode = AUTOTUNE_MODE_UNINITIALISED;
     }
 }