net: dwmac_socfpga: initialize local data for mdio regmap configuration

Explicitly zero-ize the local mdio_regmap_config data, and explicitly
set the .autoscan parameter, as we only have a PCS on this bus.

Fixes: 5d1f3fe7 ("net: stmmac: dwmac-sogfpga: use the lynx pcs driver")
Suggested-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Suggested-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
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>