mac80211: use kfree_skb() to free struct sk_buff pointers

kfree_skb() should be used to free struct sk_buff pointers.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>