sh: pci: Support ports with disabled links on SH7786 PCIe.

Presently we error out if a link is disabled and simply drop the port
registration outright. This follows the PPC changes and simply reports on
the link state on boot, leaving the port registered, in order to more
easily deal with hotplug on future parts.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>