ath9k uses the mac80211 workqueue for 4 different types of work:

 * Led blink work
 * TX hang monitoring work
 * internal wiphy schedular work
 * channel change work done for internal wiphy schedular

Since the internal wiphy schedular can end up kicking off some
channel channel change work we should first cancel the wiphy
schedular work and then the channel change work.

The TX hang work can be cancelled second since we're going down
anyway.
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>