iwlwifi: note that priv->bssid is used only by 3945

The bssid member of struct iwl_priv is now
only used by 3945 code, so note that. It
shouldn't be used by any other code in the
future.

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