CVE-2025-68176

In the Linux kernel, the following vulnerability has been resolved:

PCI: cadence: Check for the existence of cdns_pcie::ops before using it

cdns_pcie::ops might not be populated by all the Cadence glue drivers. This
is going to be true for the upcoming Sophgo platform which doesn't set the
ops.

Hence, add a check to prevent NULL pointer dereference.

[mani: reworded subject and description]