net: dsa: qca: ar9331: make proper initial port defaults

Make sure that all external port are actually isolated from each other,
so no packets are leaked.

Fixes: ec6698c2

 ("net: dsa: add support for Atheros AR9331 built-in switch")
Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>