iwlwifi: cancel scan when down the device

Always cancel scan when stooping device and scan is currently pending,
we should newer have scan running after down device.

To assure we start scan cancel from restart work we have to schedule
abort_scan to different workqueue than priv->workqueue.

Patch fix not cancel scanning when restarting firmware, what is
one of the causes of wdev_cleanup_work warning (together with permanent
network connection lost) reported at
https://bugzilla.redhat.com/show_bug.cgi?id=593566

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Wey-Yi W Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>