mac80211: implement VHT support for mesh

Implement the basics required for supporting very high throughput
with mesh: include VHT information elements in beacons, probe
responses, and peering action frames, and check for compatible VHT
configurations when peering.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>