iwlwifi: manage IBSS station properly

Currently iwlwifi will eventually exhaust the station
table when adding the BSSID station for IBSS mode,
unless the interface is set down.

The new mac80211 ibss joined/left notification allows
us to fix that easily by moving the code to add the
IBSS station to the notification, and also adding
code to remove it again when we leave the IBSS.

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