net: dsa: add slave VLAN helpers

Add dsa_slave_vlan_add and dsa_slave_vlan_del helpers to handle
SWITCHDEV_OBJ_ID_PORT_VLAN switchdev objects. Also copy the
switchdev_obj_port_vlan structure on add since we will modify it in
future patches.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>