iwlwifi: mvm: don't use highest rate in VHT MCS Set

iwlwifi: mvm: don't use highest rate in VHT MCS Set

Keeping this as 0 is ok according to spec section 9.7.11
as this means the limits are according to the Tx/Rx
supported MCS x NSS bitmap. Initially we've set these as
there were concerns of interop issues but these turned out
to be false.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Reviewed-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>