wifi: mac80211: correctly identify S1G short beacon

mac80211 identifies a short beacon by the presence of the next
TBTT field, however the standard actually doesn't explicitly state that
the next TBTT can't be in a long beacon or even that it is required in
a short beacon - and as a result this validation does not work for all
vendor implementations.

The standard explicitly states that an S1G long beacon shall contain
the S1G beacon compatibility element as the first element in a beacon
transmitted at a TBTT that is not a TSBTT (Target Short Beacon
Transmission Time) as per IEEE80211-2024 11.1.3.10.1. This is validated
by 9.3.4.3 Table 9-76 which states that the S1G beacon compatibility
element is only allowed in the full set and is not allowed in the
minimum set of elements permitted for use within short beacons.

Correctly identify short beacons by the lack of an S1G beacon
compatibility element as the first element in an S1G beacon frame.

Fixes: 9eaffe50 ("cfg80211: convert S1G beacon to scan results")
Signed-off-by: Simon Wadsworth <simon@morsemicro.com>
Signed-off-by: Lachlan Hodges <lachlan.hodges@morsemicro.com>
Link: https://patch.msgid.link/20250701075541.162619-1-lachlan.hodges@morsemicro.com


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