s390/pci: adaptation of iommu to multifunction

In the future the bus sysdata may not directly point to the
zpci_dev.

In preparation of upcoming patches let us abstract the
access to the zpci_dev from the device inside the pci device.

Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
Reviewed-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>