PCI: imx6: Add PCIE_PHY_RX_ASIC_OUT_VALID definition

Add a #define for PCIE_PHY_RX_ASIC_OUT_VALID and use it instead of a
hardcoded value.

[bhelgaas: drop PCIE_PHY_DEBUG_R0_LTSSM_MASK; updated in future patch]
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>