s390/pci: Fix missing check for zpci_create_device() error return

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

s390/pci: Fix missing check for zpci_create_device() error return

The zpci_create_device() function returns an error pointer that needs to

be checked before dereferencing it as a struct zpci_dev pointer. Add the

missing check in __clp_add() where it was missed when adding the

scan_list in the fixed commit. Simply not adding the device to the scan

list results in the previous behavior.