net80211: VHT correct NSS Set loop boundary

For the <VHT-MCS, NSS> tuple, NSS is 1..8 (or in our loop case 0..7
but not 0..6). Correct the boundry to check for < 8 and not < 7.

MFC after: 2 weeks
Reviewed by: adrian
Sponsored by: Rubicon Communications, LLC (d/b/a "Netgate")
Differential Revision: https://reviews.freebsd.org/D26087