PCI: imx6: Return real error code from imx6_add_pcie_port()

When devm_request_irq() fails, imx6_add_pcie_port() should return the real
error code instead of always returning -ENODEV.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Bjorn Helgaas <helgaas@kernel.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>