PPC: 440: Ignore invalid PCI IRQs

When running a 440 target, we currently get invalid irq_num values (-1)
which completely confuse the IRQ setting code.

This is most likely due to the missing qdev conversion.

While this shouldn't happen in the first place and should really rather
be fixed by converting the target, I dislike segfaults. So for now, let's
just print a warning and ignore invalid irq_num values.

Signed-off-by: Alexander Graf <agraf@suse.de>
