mavlink: fix warning on do jump

This fixes the warning:
Mission storage: Unable to read from microSD
which appeared on do jump mission items. The reason was that the
_mission_type can be set to rally or geofence if that was the last
transfer which had happened earlier. Therefore, it would then try to
read a certain rally item from dataman when really a mission item was
supposed to be read.

src/modules/mavlink/mavlink_mission.cpp

@@ -499,6 +499,7 @@ MavlinkMissionManager::send(const hrt_abstime now)
 
 		if (mission_result.item_do_jump_changed) {
 			/* send a mission item again if the remaining DO_JUMPs has changed */
+			_mission_type = MAV_MISSION_TYPE_MISSION;
 			send_mission_item(_transfer_partner_sysid, _transfer_partner_compid,
 					  (uint16_t)mission_result.item_changed_index);
 		}