cxgb3 - avoid deadlock with mac watchdog

Fix a deadlock when the interface s configured down and
the watchdog tack is sleeping on rtnl_lock.

Signed-off-by: Divy Le Ray <[e-mail]>
Signed-off-by: Jeff Garzik <[e-mail]>

drivers/net/cxgb3/cxgb3_main.c

···
 {
 	int i;
 
-	rtnl_lock();                      /* synchronize with ifdown */
+	if (!rtnl_trylock())	/* synchronize with ifdown */
+		return;
+
 	for_each_port(adapter, i) {
 		struct net_device *dev = adapter->port[i];
 		struct port_info *p = netdev_priv(dev);