mac80211: track enable_beacon explicitly

mac80211: track enable_beacon explicitly

Instead of calculating in ieee80211_bss_info_change_notify()
whether beaconing should be enabled or not, set it in the
correct places in the callers. This simplifies the logic in
this function at the expense of offchannel, but is also more
robust.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>