s390/pci: only add specific device in zpci_bus_scan_device()

As the name suggests zpci_bus_scan_device() is used to scan a specific
device and thus pci_bus_add_device() for that device is sufficient.
Furthermore move this call inside the rescan/remove locking.

Suggested-by: Bjorn Helgaas <helgaas@kernel.org>
Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Link: https://lore.kernel.org/r/20230306151014.60913-3-schnelle@linux.ibm.com


Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>