iwlagn: use iwl_sta_id() for aggregation

With the station ID being stored in the
station struct, which mac80211 gives us
for aggregation callbacks, we can also
remove the use of iwl_find_station() in
those code paths.

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