net80211: update ieee80211_node_get_txrate_mbit() to support VHT

Update ieee80211_node_get_txrate_mbit() to use
ieee80211_phy_vht_get_mcs_kbit() to calculate VHT rates.

For now just assume long-GI; I'd have to go and do some extra
work to figure out which short/long GI flag to use.

Differential Revision: https://reviews.freebsd.org/D48613