b43: remove get_tx_stats() mac80211 op

get_tx_stats() will be removed from mac80211.

Compile-tested only.

Cc: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Michael Buesch <mb@bu3sch.de>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>