net: dsa: microchip: ksz8: Separate static MAC table operations for code reuse

Move static MAC table operations to separate functions in order to reuse
the code for add/del_fdb. This is needed to address kernel warnings
caused by the lack of fdb add function support in the current driver.

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