lan78xx: Do not access skb_queue_head list pointers directly.

Use skb_queue_walk() instead.

Adjust inner loop test to utilize and skb_queue_is_first().
Unfortunately we have to keep pkt_cnt around because it is
used by a latter loop in this function.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/usb/lan78xx.c

@@ -3340,9 +3340,9 @@ static void lan78xx_tx_bh(struct lan78xx_net *dev)
 	count = 0;
 	length = 0;
 	spin_lock_irqsave(&tqp->lock, flags);
-	for (skb = tqp->next; pkt_cnt < tqp->qlen; skb = skb->next) {
+	skb_queue_walk(tqp, skb) {
 		if (skb_is_gso(skb)) {
-			if (pkt_cnt) {
+			if (!skb_queue_is_first(tqp, skb)) {
 				/* handle previous packets first */
 				break;
 			}