PCI: imx6: Mark the link down as non-fatal error

If the PCIe link is down, return zero from imx6_pcie_start_link() so the
driver will probe successfully.

Link: https://lore.kernel.org/r/1657783869-19194-13-git-send-email-hongxing.zhu@nxp.com


Signed-off-by: Richard Zhu <hongxing.zhu@nxp.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>