ath9k_htc: Drain pending TX frames properly

When doing a channel set or a reset operation the pending
frames queued up for transmission have to be flushed and
sent to mac80211. Fixing this has to be done in two separate
steps:

  * Flush queued frames and kill the URB TX completion handler.
  * Complete all the frames that in the TX pending queue.

This patch adds proper support for draining and all the callsites
namely, channel change/reset/idle/stop are fixed. A separate queue
is used for handling failed frames.

Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>