net: dsa: microchip: add switch offload forwarding support

The flag offload_fwd_mark is set as the switch can forward frames by
itself.

This can be considered a fix to a problem introduced in commit
c2e86691 where the port membership are not set in sync.  The flag
offload_fwd_mark just needs to be set in tag_ksz.c to prevent the software
bridge from forwarding duplicate multicast frames.

Fixes: c2e86691 ("microchip: break KSZ9477 DSA driver into two files")
Signed-off-by: Tristram Ha <Tristram.Ha@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>