net80211: add VHT MCS in AMRR rate control

This is a simple implementation, similar to HT, to generate
VHT MCS rates for testing.

I've verified it seems to do the right thing through
MCS 0..9 and NSS 1..2. It's not very optimal - same issues as
with 11n - but it at least now fully tests the 11ac TX path in
rtwn and the tx rate refactoring.

Differential Revision: https://reviews.freebsd.org/D48611