net: macb: avoid redundant lookup for "mdio" child node in MDIO setup

Pass the "mdio" child node directly to `macb_mdiobus_register` to avoid
performing the node lookup twice.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20241030085224.2632426-1-o.rempel@pengutronix.de


Signed-off-by: Jakub Kicinski <kuba@kernel.org>