Bluetooth: mgmt: multi adv for create_instance_adv_data()

The create_instance_adv_data() function could not deal with
multiple advertising instances previously. This is being fixed by
retrieving advertising instances from the newly introduced dynamic
advertising instance list.

Signed-off-by: Florian Grandel <fgrandel@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>