iwlwifi: mvm: rs: fix a WARNING in case of STBC and VHT

This was taken care of in case we're doing STBC with HT
but not when working with VHT.

Signed-off-by: Eyal Shapira <eyalx.shapira@intel.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>