Bluetooth: mgmt: multi adv for get_adv_instance_flags()

The get_adv_instance_flags() would not work with instance identifiers
other than 0x01. This is being fixed so that arbitrary instance
identifiers can be dealt with while still correctly dealing with the
special case of the 0x00 identifier.

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