mission_raw: fix upload race condition

This fixes the case where a rally points upload right after a geofence
upload fails with busy. That's because the work item still exists and
hasn't been cleaned up yet.

However, we can ignore that case if the work item has been done.

src/mavsdk/plugins/mission_raw/mission_raw_impl.cpp

@@ -134,7 +134,8 @@ void MissionRawImpl::upload_mission_items_async(
     uint8_t type,
     const MissionRaw::ResultCallback& callback)
 {
-    if (_last_upload.lock()) {
+    auto work_item = _last_upload.lock();
+    if (work_item && !work_item->is_done()) {
         _system_impl->call_user_callback([callback]() {
             if (callback) {
                 callback(MissionRaw::Result::Busy);
@@ -227,7 +228,8 @@ MissionRawImpl::download_mission()
 
 void MissionRawImpl::download_mission_async(const MissionRaw::DownloadMissionCallback& callback)
 {
-    if (_last_download.lock()) {
+    auto work_item = _last_download.lock();
+    if (work_item && !work_item->is_done()) {
         _system_impl->call_user_callback([callback]() {
             if (callback) {
                 std::vector<MissionRaw::MissionItem> empty_items;