In ieee80211_sta_join() there are currently two ways to set
"do_ht": (1) after checking HT IEs are avail, and (2) after
checking VHT IEs are avail and we are not on 2GHz.

In the latter case no one checks that HT IEs are available and
when we hit ieee80211_ht_updateparams_final() htinfo may be NULL
and we panic.

Avoid this by only checking for VHT if do_ht was set.
No VHT without HT IEs.

While here switch do_ht to be a bool.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
PR: 287625
Fixes: 51172f62a753f