net80211: LinuxKPI 802.11: harmonize IEEE80211_VHT_MCS_*

Rather than defining the same values in two places and having to do
conflict resulution on the name in LKPI, change the defines to an
enum in net80211. In addition to de-duplication this also gives us
value checks in certain cases.

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D36250