powerpc/PCI: Use PCI_UNKNOWN for unknown power state

Previously we initialized dev->current_state to 4 (PCI_D3cold), but I think
we wanted PCI_UNKNOWN (5) here based on the comment and the fact that the
generic version of this code, pci_setup_device(), uses PCI_UNKNOWN.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>