PCI: Always define the of_node helpers

Simply move these inline functions outside the ifdef instead of duplicating
them as stubs in the !OF case.  The struct device of_node field does not
depend on OF.

This also fixes the missing stubbed pci_bus_to_OF_node().

Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>