net80211: 11ac: add options to manage VHT STBC

[bz]

Summary

Add IEEE80211_FVHT_STBC_(TX|RX) flags to allow userspace to manage
if STBC will be allowed for RX/TX.
For RX this only will allows 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

[bz]

sys/net80211/ieee80211.c

748

Still not sure if we really need to sync them to the ic here (and below).