Bluetooth: Perform Class of Device changes through hdev->req_workqueue

The Class of Device needs to be changed e.g. for limited discoverable
mode. In preparation of moving the discoverable mode to hci_request.c
and hdev->req_workqueue, move the Class of Device helpers there first.

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