p54: remove get_tx_stats() mac80211 op

get_tx_stats() will be removed from mac80211.

p54 uses struct ieee80211_tx_queue_stats also internally, so create a new
identical struct p54_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>