ptp: ocp: Fix a couple NULL vs IS_ERR() checks

The ptp_ocp_get_mem() function does not return NULL, it returns error
pointers.

Fixes: 773bda96

 ("ptp: ocp: Expose various resources on the timecard.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>