net: dsa: uninitialized variable in dsa_slave_netdevice_event()

Return zero if both dsa_slave_dev_check() and netdev_uses_dsa() are false.

Fixes: acc43b7b ("net: dsa: allow masters to join a LAG")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>