s390/PCI: Fix single MSI only check

Multiple MSIs have never been supported on s390 architecture, but the
platform code fails to report single MSI only.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Martin Schwidefsky <schwidefsky@de.ibm.com>