net80211: rename iv_flags_vht to iv_vht_flags

While the flag field is internal start naming it as well as "iv_vht*"
so we keep all "VHT" fields together. This breaks with what was done
done for HT but with HE, EHT, .. coming one day seems the more logic
choice.

No functional changes intended.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian, cc
Differential Revision: https://reviews.freebsd.org/D42419