iwlwifi: clear driver stations when going down

During a hw restart, mac80211 will attempt to
reconfigure all stations. Currently, that fails
and leads to warnings because we still have the
stations marked active. Therefore, clear all
stations when doing down.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>