net: mdio: realtek-rtl9300: harden otto_emdio_probe_one()

net: mdio: realtek-rtl9300: harden otto_emdio_probe_one()

The bus probing of the MDIO driver uses a two stage approach.

1. The device tree "ethernet-ports" node is scanned to build a mapping
   between ports and PHYs.

2. The children of the device tree "controller" are scanned to create
   the individual MDIO buses.

The first step already checks the consistency of the PHY and bus nodes
that are linked via the ports. But it might miss a dangling bus child
node that is not linked. Step two simply iterates over all bus child
nodes and might read malformed data from nodes not checked in step one.

Harden this and return a meaningful error message.

Signed-off-by: Markus Stockhausen <markus.stockhausen@gmx.de>
Link: https://patch.msgid.link/20260603175924.123019-4-markus.stockhausen@gmx.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>