s390/pci: introduce zpci_bus_scan_device()

To match zpci_bus_scan_device() and the PCI common code terminology and
to remove some code duplication, we pull the multiple uses of
pci_scan_single_device() into a function. For now this has the side
effect of adding each device to the PCI bus separately and locking and
unlocking the rescan/remove lock for each instead of just once per bus.
This is clearly less efficient but provides a correct intermediate
behavior until a follow on change does both the adding and scanning only
once per bus.

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