iwlwifi: mvm: set TWT responder capability bit in 11AX SoftAP mode

This is needed to test TWT requester on the client side.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>