cxgb4: when disabling dcb set txq dcb priority to 0

When we are disabling DCB, store "0" in txq->dcb_prio
since that's used for future TX Work Request "OVLAN_IDX"
values. Setting non zero priority upon disabling DCB
would halt the traffic.
Reported-by: NAMG Zollner Robert <robert@cloudmedia.eu>
CC: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NCasey Leedom <leedom@chelsio.com>
Signed-off-by: NGanesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c

@@ -263,7 +263,7 @@ static void dcb_tx_queue_prio_enable(struct net_device *dev, int enable)
 				"Can't %s DCB Priority on port %d, TX Queue %d: err=%d\n",
 				enable ? "set" : "unset", pi->port_id, i, -err);
 		else
-			txq->dcb_prio = value;
+			txq->dcb_prio = enable ? value : 0;
 	}
 }