[POWERPC] Fix PCI IRQ fallback code to not map IRQ 0

The PCI IRQ code has a fallback when the device-tree parsing fails, that
tries to map the interrupt indicated by PCI_INTERRUPT_LINE if the firmware
set something in there. This is a bit fragile but has proven useful in some
cases so far. However, it's causing us to incorrectly try to map interrupt 0
on various setups, so let's prevent that case, as none of the cases where
the fallback is legit should have an IRQ 0.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>