net: dsa: microchip: ksz8: Prepare ksz8863_smi for regmap register access validation

This patch prepares the ksz8863_smi part of ksz8 driver to utilize the
regmap register access validation feature.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>