iwlwifi: make WEP key restoring explicit

The firmware clears default WEP keys on
transitions to !associated, so we need
to restore them just like stations. This
is rather implicit as part of sending a
station right now, which is odd. Make it
explicit instead and only for agn since
3945 doesn't use hw crypto for WEP.

Due to that, iwl_send_static_wepkey_cmd
is now only used in iwl-sta.c and can be
static.

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