mavlink_mission: set INT frame correctly

Also when formatting malvink mission items from internal mission items,
the INT flag should be obeyed.

src/modules/mavlink/mavlink_mission.cpp

@@ -1138,10 +1138,20 @@ MavlinkMissionManager::format_mavlink_mission_item(const struct mission_item_s *
 		mavlink_mission_item->z = mission_item->altitude;
 
 		if (mission_item->altitude_is_relative) {
-			mavlink_mission_item->frame = MAV_FRAME_GLOBAL_RELATIVE_ALT;
+			if (_int_mode) {
+				mavlink_mission_item->frame = MAV_FRAME_GLOBAL_RELATIVE_ALT_INT;
+
+			} else {
+				mavlink_mission_item->frame = MAV_FRAME_GLOBAL_RELATIVE_ALT;
+			}
 
 		} else {
-			mavlink_mission_item->frame = MAV_FRAME_GLOBAL;
+			if (_int_mode) {
+				mavlink_mission_item->frame = MAV_FRAME_GLOBAL_INT;
+
+			} else {
+				mavlink_mission_item->frame = MAV_FRAME_GLOBAL;
+			}
 		}
 
 		switch (mission_item->nav_cmd) {