iwlwifi: track station IDs

mac80211 allows us to store private data per
station, so put the station ID there. This
allows us to avoid the station ID lookup when
removing regular stations. To also be able to
avoid the lookup to remove the special IBSS
BSSID station, track its ID in the per-vif
private data.

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