If the device is MIMO and VHT capable it supports Tx STBC.
Unlike HT, any chip that supports VHT also support STBC so
no need for a config parameter.
Using num_of_ants multiple times warranted caching it.
Signed-off-by: Eyal Shapira <eyal@wizery.com>
Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>