DSA: Enable cascading in multi-chip 6131 configuration

This patch enables the 6131 family of chips to forward DSA
packets to other switch chips.  This is needed if multiple
DSA chips are used in a device.  Without this patch the
chip will drop any DSA packets not destined for it.

This patch only enables the forwarding of DSA packets if
multiple chips are used in the switch configuration.

Signed-off-by: Barry Grussling <barry@grussling.com>
Signed-off-by: David S. Miller <davem@davemloft.net>