cfg80211/nl80211: add beacon settings

This adds the necessary API to cfg80211/nl80211 to allow
changing beaconing settings.

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