get_tx_stats() will be removed from mac80211.

Because ar9170 uses ieee80211_tx_queue_stats internally, create a new
identical struct ar9170_tx_queue_stats which the driver can use.

Compile-tested only.
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Tested-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>