wifi: iwlwifi: Correctly indicate support for VHT TX STBC

If HT STBC is not supported, do not indicate support for VHT TX
STBC.

Signed-off-by: Ilan Peer <ilan.peer@intel.com>
Signed-off-by: Gregory Greenman <gregory.greenman@intel.com>
Link: https://lore.kernel.org/r/20230614123446.b24b5fba6fab.I116617875eb4a9d520df23a8c49a6594f9d8b2c6@changeid


Signed-off-by: Johannes Berg <johannes.berg@intel.com>