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.

Cc: stable@vger.kernel.org
Fixes: 0467cdde ("s390/pci: Sort PCI functions prior to creating virtual busses")
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Gerd Bayer <gbayer@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>