ath9k: get rid of tx_info_priv

This patch removes the need for separately allocated private tx info
data in ath9k and brings the driver one small step closer to using the
mac80211 rate control API properly.

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