ptp: Set lookup cookie when creating a PTP PPS source.

When creating a PTP device, the configuration block allows
creation of an associated PPS device.  However, there isn't
any way to associate the two devices after creation.

Set the PPS cookie, so pps_lookup_dev(ptp) performs correctly.

Signed-off-by: Jonathan Lemon <jonathan.lemon@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>