iwlwifi: rewrite scan completion

Assure (partially) we call ieee80211_scan_completed() only once when
scan was requested from mac80211.

Code path that first clear STATUS_SCANNING bit is responsible to call
ieee80211_scan_completed(). Before the call, we check if mac80211
really request the scan.

Still persist some cases when we behave wrong, that will be addressed
in the next patches.

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>