net80211: combine iv_vhtcaps and iv_vht_mcsinfo

The iv_vhtcaps and iv_vht_mcsinfo fields together form
struct ieee80211_vht_cap so combine them into one field in the VAP
and keep the information together.

While the structure layout changes no other functional changes intended.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian, cc
Differential Revision: https://reviews.freebsd.org/D42420