Drivers must be ready to accept NULL from ptp_clock_register() if the
PTP clock subsystem is configured out.

This patch documents that and ensures that all drivers cope well
with a NULL return.
Signed-off-by: NNicolas Pitre <nico@linaro.org>
Reviewed-by: NEugenia Emantayev <eugenia@mellanox.com>
Acked-by: NRichard Cochran <richardcochran@gmail.com>
Acked-by: NEdward Cree <ecree@solarflare.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>