iwlwifi: introduce iwl_sta_id_or_broadcast

There are now five places where we need to
look up the station ID, but the sta pointer
may be NULL due to mac80211 passing that to
indicate a certain special state.

Replace all these by a new inline function,
called iwl_sta_id_or_broadcast(), and add
documentation about when to use it.

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