ath9k: use ieee80211_free_txskb

commit 249ee72249140fe5b9adc988f97298f0aa5db2fc upstream.

Using ieee80211_free_txskb for tx frames is required, since mac80211 clones
skbs for which socket tx status is requested.

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>