net: dsa: microchip: fix kernel oops on ksz8 switches

After driver refactoring we was running ksz9477 specific CPU port
configuration on ksz8 family which ended with kernel oops. So, make sure
we run this code only on ksz9477 compatible devices.

Tested on KSZ8873 and KSZ9477.

Fixes: da8cd085

 ("net: dsa: microchip: add support for common phylink mac link up")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>