r8169: fix sporadic transmit timeout issue

Holger reported sporadic transmit timeouts and it turned out that one
path misses ringing the doorbell. Fix was suggested by Eric.

Fixes: ef143585 ("r8169: make use of xmit_more")
Suggested-by: Eric Dumazet <edumazet@google.com>
Tested-by: Holger Hoffstätte <holger@applied-asynchrony.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/realtek/r8169_main.c

@@ -5681,6 +5681,7 @@ static netdev_tx_t rtl8169_start_xmit(struct sk_buff *skb,
 		 */
 		smp_wmb();
 		netif_stop_queue(dev);
+		door_bell = true;
 	}
 
 	if (door_bell)