Bluetooth: mgmt: multi adv for set_advertising*()

The set_advertising() and set_advertising_complete() methods rely on
the now obsolete hci_dev->adv_instance structure. We replace this
reference by an equivalent access to the newly introduced dynamic
advertising instance list.

This patch introduces a helper function that schedules an advertising
instance correctly calculating advertising timing based on the timeout
and duration settings of the instance. Scheduling is factored into
its own function for readability and code sharing.

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