net: dsa: dsa_loop: remove usage of mdio_board_info

dsa_loop is the last remaining user of mdio_board_info. Let's remove
using mdio_board_info, so that support for it can be dropped from
phylib.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Tested-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/da9563a4-8e14-41cf-bfea-cf5f1b58a4b7@gmail.com


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