net: ibm: emac: Use of_get_available_child_by_name()

Use the helper of_get_available_child_by_name() to simplify
emac_dt_mdio_probe().

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>