net80211: Implement ieee80211_setup_vht_rates()

Implement ieee80211_setup_vht_rates() - calculate the intersection between
the peers advertised RX VHT MCS map and the vap's configured TX map.

Whilst here, remove the unused vhtcap/vhtinfo fields; they're
already populated in the ieee80211_node before ieee80211_setup_vht_rates()
is called.

Differential Revision: https://reviews.freebsd.org/D48607
Reviewed by: bz