PCI: layerscape: Ignore PCIe controllers in Endpoint mode

Layerscape PCIe controller supports root complex (RC) and endpoint (EP)
modes, which can be set by RCW.

If not in RC mode, return -ENODEV without claiming the controller.

Signed-off-by: Minghuan Lian <Minghuan.Lian@freescale.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>