powerpc/powernv: Use pci_dn, not device_node, in PCI config accessor

The PCI config accessors previously relied on device_node.  Unfortunately,
VFs don't have a corresponding device_node, so change the accessors to use
pci_dn instead.

[bhelgaas: changelog]
Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>