mdio node is optional for the rtl836x switch. Some devices use GPIO to
control the switch chip. Missing mdio is a normal behavior.
Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Link: https://github.com/openwrt/openwrt/pull/21143
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>