of/pci: Fix theoretical NULL dereference

In the (relatively mechanical) process of adapting the RID-mapping code
to put the resulting ID in an output argument rather than the funtion
return value, we ended up with the debug print using the argument
pointer rather than the local value, which potentially defeats the
earlier NULL check.

Fixes: 987068fc: "of/irq: Break out msi-map lookup (again)"
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Rob Herring <robh@kernel.org>