nl80211: use attributes to parse beacons

only the attributes are required and not the whole netlink info, as the
function accesses the attributes only anyway. This makes it easier to
parse nested beacon IEs later.

Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Mathias Kretschmer <mathias.kretschmer@fokus.fraunhofer.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>