net80211: add valid VHT MCS combinations and helper functions

Not all NSS / MCS / channel bandwidth combinations are valid.

This will be important for rate control and transmit rate
setup (eg static management, unicast, multicast rates.)

This is based on 802.11-2020 Section 21.5 (Parameters for VHT-MCSs.)

Differential Revision: https://reviews.freebsd.org/D48608
Reviewed by: bz