net: dsa: microchip: lan937x: return zero if mdio node not present

Currently, if the mdio node is not present in the dts file then
lan937x_mdio_register return -ENODEV and entire probing process fails.
To make the mdio_register generic for all ksz series switches and to
maintain back-compatibility with existing dts file, return -ENODEV is
replaced with return 0.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>