pci: imx: Adjust the return value when imx_pcie_addr_valid() fails

pci: imx: Adjust the return value when imx_pcie_addr_valid() fails

When trying to access non-existent/unsupported PCI devices in
imx_pcie_read_config(), when imx_pcie_addr_valid() fails it returns
error code and fills in the result with 0xffffffff manually. But it
really should return zero to upper layer codes.

Reported-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Fabio Estevam <fabio.estevam@nxp.com>