net: dsa: microchip: ksz8_r_sta_mac_table(): Utilize error values from read/write functions

Take advantage of the return values provided by read/write functions in
ksz8_r_sta_mac_table() to handle cases where read/write operations may
fail.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>