net80211 / ifconfig: cleanup the use of IEEE80211_FVHT_USEVHT*

Rather then using magic numbers duplicate IEEE80211_FVHT_VHT* in
ifconfig (cleanup of these and other flags used and not exposed by
net80211 should happen later) and use those.
In the kernel this simplifies one ioctl path (the other one currently
relies on individual bit flags being passed in).
We also re-order the 80P80 and 160 flag for 160 to come before 80+80
and more clearly leave the flags as TODO in one of the 160/80+80 cases.

Reviewed by: adrian
MFC after: 2 weeks
Sponsored by: Rubicon Communications, LLC (d/b/a "Netgate")
Differential Revision: https://reviews.freebsd.org/D26000