wifi: cfg80211: correct S1G beacon length calculation

The minimum header length calculation (equivalent to the start
of the elements) for the S1G long beacon erroneously required
only up to the start of u.s1g_beacon rather than the start of
u.s1g_beacon.variable. Fix that, and also shuffle the branches
around a bit to not assign useless values that are overwritten
later.

Reported-by:  <syzbot+0f3afa93b91202f21939@syzkaller.appspotmail.com>
Fixes: 9eaffe50 ("cfg80211: convert S1G beacon to scan results")
Link: https://patch.msgid.link/20240724132912.9662972db7c1.I8779675b5bbda4994cc66f876b6b87a2361c3c0b@changeid


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