iommu/s390: Fail probe for non-PCI devices

s390-iommu only supports pci_bus_type today.

Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/8cb71ea1b24bd2622c1937bd9cfffe73b126eb56.1660572783.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>