ar9170: cleanup of bss_info_changed and beacon config

Add beacon control by BSS_CHANGED_BEACON_ENABLED and
bss_conf->enable_beacon from mac80211.

Signed-off-by: Joerg Albert <jal2@gmx.de>
Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>