s390/pci: PCI function group 0 is valid for clp_query_pci_fn

The PCI function group 0 is a valid function group,
it is wrong to reject it.

Let's accept PCI function group 0.

Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
Acked-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>