mac80211: convert S1G beacon to scan results

This commit finds the correct offset for Information
Elements in S1G beacon frames so they can be reported in
scan results.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-8-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>