ftp: implement progress tracking

Make sure FTP_STATE_FINISHED is returned for transactions that
are marked 'done'.

This is necessary for timely logging and inspection.

src/app-layer-ftp.c

@@ -1014,9 +1014,10 @@ static int FTPGetAlstateProgress(void *vtx, uint8_t direction)
         return FTP_STATE_PORT_DONE;
     }
 
-    /* TODO: figure out further progress handling */
+    if (!tx->done)
+        return FTP_STATE_IN_PROGRESS;
 
-    return FTP_STATE_IN_PROGRESS;
+    return FTP_STATE_FINISHED;
 }