at76c50x-usb: cancel scan work at stop callback

This should fix suspend as mac80211 expects all work queued
to the mac80211 workqueue to be canceled at driver stop().

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>