net80211: migrate ic_vhtcaps, ic_vht_mcsinfo, ic_flags_vht

Like for the VAP rename ic_flags_vht to ic_vht_flags for consistency to
keep "VHT" fields together and merge ic_vhtcaps and ic_vht_mcsinfo
into struct ieee80211_vht_cap ic_vht_cap.

While the structure layout changes no other functional changes intended.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian, cc
Differential Revision: https://reviews.freebsd.org/D42421

(cherry picked from commit 562adbe1d354377a260e66eedb6072b720a606dc)