commander: move valid mission check inside

I think it makes more sense to move this check inside of
main_state_transition.

src/modules/commander/Commander.cpp

@@ -813,13 +813,7 @@ Commander::handle_command(const vehicle_command_s &cmd)
 							break;
 
 						case PX4_CUSTOM_SUB_MODE_AUTO_MISSION:
-							if (_status_flags.condition_auto_mission_available) {
+							desired_main_state = commander_state_s::MAIN_STATE_AUTO_MISSION;
-								desired_main_state = commander_state_s::MAIN_STATE_AUTO_MISSION;
-
-							} else {
-								main_ret = TRANSITION_DENIED;
-							}
-
 							break;
 
 						case PX4_CUSTOM_SUB_MODE_AUTO_RTL:

src/modules/commander/state_machine_helper.cpp

@@ -368,9 +368,13 @@ main_state_transition(const vehicle_status_s &status, const main_state_t new_mai
 
 	case commander_state_s::MAIN_STATE_AUTO_MISSION:
 
+		if (!status_flags.condition_auto_mission_available) {
+			ret = TRANSITION_DENIED;
+		}
+
 		/* need global position, home position, and a valid mission */
-		if (status_flags.condition_global_position_valid &&
+		else if (status_flags.condition_global_position_valid &&
-		    status_flags.condition_auto_mission_available) {
+			 status_flags.condition_auto_mission_available) {
 
 			ret = TRANSITION_CHANGED;
 		}