net: dsa: microchip: allow to pass return values for PHY read/write accesses

PHY access may end with errors on different levels. So, allow to forward
return values where possible.

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>