net: altera-tse: Initialize local structs before using it

The regmap_config and mdio_regmap_config objects needs to be zeroed before
using them. This will cause spurious errors at probe time as config->pad_bits
is containing random uninitialized data.

Fixes: db48abba ("net: ethernet: altera-tse: Convert to mdio-regmap and use PCS Lynx")
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>