ath5k: fix tx path skb leaks

commit 596ab5ec3bf10a22be30d7cb1d903a4b83fd607c upstream.

ieee80211_free_txskb() needs to be used instead of dev_kfree_skb_any for
tx packets passed to the driver from mac80211

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>