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