rtwn: announce VHT support for RTL8812AU/RTL8821AU.

Although the transmit path doesn't yet support VHT rates (because
the rate control and rate representation in net80211 doesn't yet
know about VHT rates) the NIC will receive VHT frames but only
transmit HT frames.

Locally tested:

• RTL8812AU, STA mode

Differential Revision: https://reviews.freebsd.org/D48103