net: dsa: microchip: ksz8_r_dyn_mac_table(): use entries variable to signal 0 entries

We already have a variable to provide number of entries. So use it,
instead of using error number.

Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://lore.kernel.org/r/20240403125039.3414824-9-o.rempel@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>