mac80211: fix VHT MCS calculation

The VHT MCSes we advertise to the AP were supposed to
be restricted to the AP, but due to a bug in the logic
mac80211 will advertise rates to the AP that aren't
even supported by the local device. To fix this skip
any adjustment if the NSS isn't supported at all.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>