s390x/pci: add checkings in CLP_SET_PCI_FN

The code in CLP_SET_PCI_FN case misses some checkings. Let's add
them.

Signed-off-by: Yi Min Zhao <zyimin@linux.vnet.ibm.com>
Reviewed-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>
