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.

Change-Id: I63a958b1545380d2f53905a7fe8ad807b8dcee39
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: e42a43a5cab2e019b5ab82bedb1340854709154d
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


[luliang@codeaurora.org: remove '.' in commit summary line]
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>