powerpc/pci/rpadlpar: Fix device reference leaks

Make sure to drop any device reference taken by vio_find_node() when
adding and removing virtual I/O slots.

Fixes: 5eeb8c63 ("[PATCH] PCI Hotplug: rpaphp: Move VIO registration")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>