Bluetooth: hci_core/mgmt: Introduce multi-adv list

The current hci dev structure only supports a single advertising
instance. To support multi-instance advertising it is necessary to
introduce a linked list of advertising instances so that multiple
advertising instances can be dynamically added and/or removed.

In a first step, the existing adv_instance member of the hci_dev
struct is supplemented by a linked list of advertising instances.
This patch introduces the list and supporting list management
infrastructure. The list is not being used yet.

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