[POWERPC] pci_32.c: Switch to ref counting PCI API

pci_find_slot isn't hot-plug safe.  Move this code to the pci hotplug
safe equivalent and hold a refcount properly while doing
make_one_node_map.

Signed-off-by: Alan Cox <alan@redhat.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>