mac80211: deprecate conf.beacon_int properly

Ivo has updated the driver to no longer use the change flag,
so we can remove that, but rt2x00 and ath5k still use the
actual value so let's mark it as deprecated too.

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