Export Dialog: correctly init export channels

The state has to be pushed to the manager during initialization,
otherwise channel_config remains unset.

This fixes an issue with files using the same filename
(missing channel name) during stem export.

gtk2_ardour/export_channel_selector.cc

@@ -647,9 +647,8 @@ TrackExportChannelSelector::~TrackExportChannelSelector ()
 void
 TrackExportChannelSelector::sync_with_manager ()
 {
-	if (sync_with_manager_state ()) {
-		update_config();
-	}
+	sync_with_manager_state ();
+	update_config();
 }
 
 bool