iwlwifi: use iwl_sta_id() for TKIP key update

With the station ID being stored in the
station struct, which mac80211 gives us
for TKIP phase 1 key updates, we can also
remove the use of iwl_find_station() in
that code path.

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