wifi: cfg80211: don't allow multi-BSSID in S1G

In S1G beacon frames there shouldn't be multi-BSSID elements
since that's not supported, remove that to avoid a potential
integer underflow and/or misparsing the frames due to the
different length of the fixed part of the frame.

While at it, initialize non_tx_data so we don't send garbage
values to the user (even if it doesn't seem to matter now.)

Reported-and-tested-by: Sönke Huster <shuster@seemoo.tu-darmstadt.de>
Fixes: 9eaffe50 ("cfg80211: convert S1G beacon to scan results")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>