iwlwifi: notify on scan completion even when shutting down

Under certain circumstances iwlwifi can get stuck and will no
longer accept scan requests, because the core code (cfg80211)
thinks that it's still processing one. This fixes one of the
points where it can happen, but I've still seen it (although
only with my radio-off-when-idle patch).

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>