cxgb4: ptp_clock_register() returns error pointers

Check ptp_clock_register() return not only for NULL but
also for error pointers, and also nullify adapter->ptp_clock
if ptp_clock_register() fails.

Fixes: 9c33e420

 ("cxgb4: Add PTP Hardware Clock (PHC) support")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>