bnxt_en: Fix xmit_more with BQL.

We need to write the doorbell if BQL has stopped the queue and
skb->xmit_more is set.  Otherwise it is possible for the tx queue to
rot and cause tx timeout.

Fixes: 4d172f21 ("bnxt_en: Implement xmit_more.")
Suggested-by: Yuval Mintz <yuval.mintz@cavium.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/broadcom/bnxt/bnxt.c

@@ -463,7 +463,7 @@ static netdev_tx_t bnxt_start_xmit(struct sk_buff *skb, struct net_device *dev)
 	prod = NEXT_TX(prod);
 	txr->tx_prod = prod;
 
-	if (!skb->xmit_more)
+	if (!skb->xmit_more || netif_xmit_stopped(txq))
 		bnxt_db_write(bp, txr->tx_doorbell, DB_KEY_TX | prod);
 
 tx_done: