ptp: do not explicitly set drvdata in ptp_clock_register()

We do not need explicitly call dev_set_drvdata(), as it is done for us by
device_create().

Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>