net80211: VHT correct check/option in ieee80211_vht_adjust_channel()

In ieee80211_vht_adjust_channel() we have to check for all possible
IEEE80211_FVHT_VHT* options using the mask rather than just checking
for IEEE80211_FVHT_VHT; ieee80211_vhtchanflags() (contrary to its
HT counterpart) only returns the "highest" flag nor or-ing them together
with the base flag. For the moment this seems to make sense as with
more width options we'd add a pyramid.

Later on, in the same function when we get VHT160 actually go and look
for VHT160 and not VHT80.

Sponsored by: The FreeBSD Foundation
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D35977

(cherry picked from commit dc32c7133921a34f3144a4b5fc4f7c1f6cddbd1f)