net80211: 11ac: add options to manage VHT STBC

Add IEEE80211_FVHT_STBC_(TX|RX) flags to allow userspace to manage
if STBC will be allowed for VHT RX/TX.
For RX this will only allow us to turn it off but no fine grained
control of the number of supported spatial streams.

Introduce IEEE80211_FVHT_CHANWIDTH_MASK as a helper to make the
spelling out of the IEEE80211_FVHT_MASK more readable.

Update ifconfig to allow setting of these flags.

Sponsored by: The FreeBSD Foundation
MFC atfer: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D47838