Bluetooth: Move advertising instance management to hci_request.c

This paves the way for eventually performing advertising changes
through the hdev->req_workqueue. Some new APIs need to be exposed from
mgmt.c to hci_request.c and vice-versa, but many of them will go away
once hdev->req_workqueue gets used.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>