mac80211: clean up get_tx_stats callback

The callback takes a ieee80211_tx_queue_stats with a contained
array of ieee80211_tx_queue_stats_data, remove the former, rename
the latter to ieee80211_tx_queue_stats and make tx_stats() take
the array directly.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>