iwlwifi: rework iwl_scan_cancel_timeout

Since on timeout version of iwl_scan_cancel procedure we can sleep,
do not have to schedule abort_scan work to begin and perform scanning,
can do this directly. Also now, as we do not queue abort_scan from
restart work anymore, we can queue abort_scan to priv->workqueue.

Don't drop mutex when waiting for scan complete. Use STATUS_HW_SCAN bit
to check if scanning is currently pending, because STATUS_SCANNING will
be cleared only with priv->mutex taken.

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>