generic: rtl836x: support defered probe on mdio-bus

On the WNDAP620, the mdio and mdc lines are controlled by
the EMAC ethernet device. This results in a hen-vs-egg problem.
The rtl8367b driver is probed before the ethernet driver and
the mdio-bus is not available yet, which caused the rtl8367b
driver to fail.

This patch changes the rtl8366_smi_probe_of() function to
return -EPROBE_DEFER if the mdio-bus lookup failed and changes
rtl8366_smi_probe()'s signature to return the error code back to
the callee, so it can propagate back to the kernel. Which, will
retry the switch probe at a later time.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>