iwlwifi: move iwl_find_station() to 4965

4965 code is the only thing that now still
needs iwl_find_station(), so move it there
and make it static. Everything else can
rely on the station data passed by mac80211.

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