commander: neutral beep on mission with warning

When a mission is uploaded we should beep consistenly:
- valid -> positive
- warn -> neutral
- invalid -> negative

Before this, it was:
- valid -> positive
- warn -> negative
- invalid -> negative

src/modules/commander/Commander.cpp

@@ -2089,7 +2089,7 @@ Commander::run()
 
 					} else if (mission_result.warning) {
 						/* the mission has a warning */
-						tune_mission_fail(true);
+						tune_mission_warn(true);
 
 					} else {
 						/* the mission is valid */