mac80211_hwsim: write TSF timestamp correctly to S1G beacon

S1G beacons are different from normal management beacons, so write
the timestamp to the correct location there.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200922022818.15855-17-thomas@adapt-ip.com


[rewrite commit message that was not useful after patch reordering]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>