net80211 / LinuxKPI 802.11: add 802.11-2016 VHT Extended NSS BW defines

Add the mask and shift for the VHT Extended NSS BW Support field.
Document them in net80211 and further related bitmasks in LinuxKPI.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D48975