iwlwifi: publish Tx STBC support in VHT

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>