mlxsw: spectrum: Forbid LAG slave from having VLAN uppers

When a port netdev is put under LAG it cannot have VLAN upper devices,
so forbid that. The LAG device itself can have VLAN upper devices.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>