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

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

BugLink: https://bugs.launchpad.net/bugs/1952579
[ Upstream commit c7521d3aa2fa7fc785682758c99b5bcae503f6be ]

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

Fixes: 773bda964921 ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>