powerpc/pci: Don't try to allocate resources that will be reassigned

When we know we will reassign all resources, trying (and failing)
to allocate them initially is fairly pointless and leads to a lot
of scary messages in the kernel log

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>