net/tcp/sendfile: notify the device driver of the availability of TX data on TCP retransmission

(as well as on sending normal TCP packets).

net/tcp/tcp_sendfile.c

@@ -307,6 +307,10 @@ static uint16_t sendfile_eventhandler(FAR struct net_driver_s *dev,
 
       dev->d_sndlen = sndlen;
 
+      /* Notify the device driver of the availability of TX data */
+
+      tcp_send_txnotify(psock, conn);
+
       /* Continue waiting */
 
       return flags;