powerpc/pci: Cosmetic cleanups of pci-common.c

This does a few cosmetic cleanups, moving a couple of things around
but without actually changing what the code does.

(There is a minor change in ordering of operations in
pcibios_setup_bus_devices but it should have no impact).

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