PCI: rockchip: Make 'return 0' more obvious in probe()

There's no way to get here with 'err != 0'.  Just return 0 to be more
obvious and prevent future changes from accidentally erroring out here
without going through the right error paths.

Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>