net80211: add ieee80211_add_vhtcap_ch()

Add an implementation of ieee80211_add_vhtcap() which works based on
information derived from the vap (and possibly channel/band but we do
not support that yet in net80211). This is needed for scans request
information in LinuxKPI at times before we have a BSS.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian, cc
Differential Revision: https://reviews.freebsd.org/D42422