mac80211: add unified BSS configuration

This patch (based on Ron Rindjunsky's) creates a framework for
a unified way to pass BSS configuration to drivers that require
the information, e.g. for implementing power save mode.

This patch introduces new ieee80211_bss_conf structure that is
passed to the driver via the new bss_info_changed() callback
when the BSS configuration changes.

This new BSS configuration infrastructure adds the following
new features:
 * drivers are notified of their association AID
 * drivers are notified of association status

and replaces the erp_ie_changed() callback. The patch also does
the relevant driver updates for the latter change.

Signed-off-by: Ron Rindjunsky <ron.rindjunsky@intel.com>
Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>