net: dsa: qca8k: add support for bridge port isolation

Remove a pair of ports from the port matrix when both ports have the
isolated flag set.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>