wil6210: use kfree_skb() instead of kfree()

Use kfree_skb() instead of kfree() to free sk_buff.

Introduced by commit e270045b
(wil6210: Sanity check for reported DMA length)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: John W. Linville <linville@tuxdriver.com>