mwl8k: remove get_tx_stats() mac80211 op

get_tx_stats() will be removed from mac80211.

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>