nl80211: correctly validate S1G beacon head

The S1G beacon has a different header size than regular
beacons, so adjust the beacon head validator.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200908190323.15814-5-thomas@adapt-ip.com


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