s390/pci: reset zdev->zbus on registration failure

On failure to register a struct zpci_dev with a struct zpci_bus we left
a dangling pointer in zdev->zbus. As zpci_create_device() bails if
zpci_bus_device_register() fails this is of no consequence but still bad
practice.

Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>