net80211: VHT correct NSS Set loop boundary

[bz]

Summary

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:
Sponsored by: Rubicon Communications, LLC (d/b/a "Netgate")

[adrian]

Oh yeah, good catch! We should also replace this with a define so we know how many max mcs streams is supported.