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
Reviewed by: adrian, cc
Differential Revision: https://reviews.freebsd.org/D42419

(cherry picked from commit ef48d4fa122d863db11e41608ceea641eec54704)