Bluetooth: Fix not sending Set Extended Scan Response

Current code is actually failing on the following tests of mgmt-tester
because get_adv_instance_scan_rsp_len did not account for flags that
cause scan response data to be included resulting in non-scannable
instance when in fact it should be scannable.

Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>