ar76c50x-usb: cancel promisc work during mac80211 stop

We weren't ever cancelling this.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Acked-by: Kalle Valo <kalle.valo@iki.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>