s390/PCI: Remove superfluous check of MSI type

arch_setup_msi_irqs() hook can only be called from the generic MSI code
which ensures correct MSI type parameter.

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