ath9k_htc: Add a timer to cleanup WMI events

Occasionally, a WMI event would arrive ahead of the TX
URB completion handler. Discarding these events would exhaust
the available TX slots, so handle them by running a timer
cleaning up such events. Also, timeout packets for which TX
completion events have not arrived.

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