iwlwifi: mvm: don't enable VHT MCS9 in 20Mhz

MCS9 in NSS=1 or NSS=2 isn't valid for 20Mhz so don't enable
it in case we're dealing with a 20Mhz sta. Trying to configure an
MCS9 rate with 20Mhz in the LQ rate table would lead to a FW assert.

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