net: dsa: microchip: ksz9477: add error handling to ksz9477_r/w_phy

Now ksz_pread/ksz_pwrite can return error value. So, make use of it.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>