HIL: when in HIL_MODE_ATTITUDE, we get altitude from gps setHIL

we need to skip the barometer and sonar calls

git-svn-id: https://arducopter.googlecode.com/svn/trunk@2891 f9c3cf11-9bcb-44bc-f272-b75c42450872

ArduCopterMega/ArduCopterMega.pde

@@ -1315,6 +1315,7 @@ static void update_alt()
 
 	#if HIL_MODE == HIL_MODE_ATTITUDE
 	current_loc.alt = g_gps->altitude;
+	return;
 	#else
 
 	if(g.sonar_enabled){