Bluetooth: mgmt: multi adv for read_adv_features()

The read_adv_features() method had a single instance identifier hard
coded. Refer to the advertising instance list instead to return a
dynamically generated list of instance identifiers.

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