mac80211: Initialize vif pointer

Before calling update_beacon() mac80211 must
initialize the control.vif pointer so it can
be used by the driver to determine which
interface is trying to send the beacon.

v2: ieee80211_beacon_get() should also initialize the
vif pointer since it can be called by mac80211 internally
before calling config_interface().

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>