mac80211: revamp beacon configuration

This patch changes mac80211's beacon configuration handling
to never pass skbs to the driver directly but rather always
require the driver to use ieee80211_beacon_get(). Additionally,
it introduces "change flags" on the config_interface() call
to enable drivers to figure out what is changing. Finally, it
removes the beacon_update() driver callback in favour of
having IBSS beacon delivered by ieee80211_beacon_get() as well.

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