ath9k: move workqueue cancels to stop callback

We should be cancelling our work at the stop callback since
we are borrowing the mac80211 workqueue for our work. As it
stands mac80211 expects this for suspend purposes.

The ath9k specific virtual wiphy stuff need only be
cancelled only when the we have no secondary virtual wiphys.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>