iwlwifi: ensure led registration complete as part of initialization

After the workqueue is notified the LED code may be accessed. Ensure
that LED registration completes completely as part of initialization
before anything waiting on this is notified.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>