ath9k_htc: Handle WMI timeouts properly

If a WMI command has timed out for some reason,
a late WMI response would end up updating the
response region of a new WMI request that has been
issued in the meantime.

Fix this race condition by dropping a WMI response
if a new WMI command has been issued.

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