drivers: pci: imx: fix enumeration logic error

By default, the subordinate is set equally to the secondary bus (1) when
the RC boots, and does not alter afterwards.

This means that theoretically, the highest bus reachable downstream is
bus 1.

Force the PCIe RC subordinate to 0xff, otherwise no downstream
devices will be detected behind bus 1 if the booting OS does not allow
enumerating a higher busnr than the subordinate value of the primary
bus.

Signed-off-by: Koen Vandeputte <koen.vandeputte@ncentric.com>