Bluetooth: mgmt: multi adv for get_current_adv_instance()

Replaces the hard coded instance identifier in
get_current_adv_instance() with the actual current instance identifier
so that this method is prepared to work with more than one advertising
instance.

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