LinuxKPI: 802.11: implement various VHT helper functions
Implement ieee80211_rate_set_vht(), ieee80211_rate_get_vht_{mcs,nss}(),
and ieee80211_get_vht_max_nss().
Sponsored by: The FreeBSD Foundation
(cherry picked from commit 5778cbdae7616c09efa6ccdd8c611d82cbc75f00)