s390/pci: allow zPCI zbus without a function zero

Currently the zPCI code blocks PCI bus creation and probing of a zPCI zbus
unless there is a PCI function with devfn 0. This is always the case for
the PCI functions with hidden RID, but may keep PCI functions from a
multi-function PCI device with RID information invisible until the function
0 becomes visible. Worse, as a PCI bus is necessary to even present a PCI
hotplug slot, even that remains invisible.

With the probing of these so-called isolated PCI functions enabled for s390
in common code, this restriction is no longer necessary. On network cards
with multiple ports and a PF per port this also allows using each port on
its own while still providing the physical PCI topology information in the
devfn needed to associate VFs with their parent PF.

Link: https://lore.kernel.org/r/20220628143100.3228092-6-schnelle@linux.ibm.com


Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Pierre Morel <pmorel@linux.ibm.com>