net: altera_tse: explicitly disable autoscan on the regmap-mdio bus

Set the .autoscan flag to false on the regmap-mdio bus, to avoid using a
random uninitialized value. We don't want autoscan in this case as the
mdio device is a PCS and not a PHY.

Fixes: db48abba

 ("net: ethernet: altera-tse: Convert to mdio-regmap and use PCS Lynx")
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>