ptp: fix NULL pointer dereference in ptp_clock_register

Fix NULL pointer dereference in ptp_clock_register. The argument
"parent" of ptp_clock_register may be NULL pointer.

Fixes: 73f37068 ("ptp: support ptp physical/virtual clocks conversion")
Reported-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>