mwl8k used struct ieee80211_tx_queue_stats internally to track the queue
lenght. Replace struct ieee80211_tx_queue_stats with a simple len field
in struct mwl8k_tx_queue. Limit and count fields seemed to be unused.
Compile-tested only.
Cc: Lennert Buytenhek <buytenh@marvell.com> Signed-off-by: Kalle Valo <kalle.valo@iki.fi> Signed-off-by: John W. Linville <linville@tuxdriver.com>