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

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]