s390/pci: skip VF scanning

Set the flag to skip scanning for VFs after SR-IOV enablement.  VF creation
will be triggered by the hotplug code.

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>