When non-bridged, non-vlan'ed mv88e6xxx port is moving down, error
message is logged:

failed to kill vid 0081/0 for device eth_cu_1000_4

This is caused by call from __vlan_vid_del() with vin set to zero, over
call chain this results into _mv88e6xxx_port_vlan_del() called with
vid=0, and mv88e6xxx_vtu_get() called from there returns -EINVAL.

On symmetric path moving port up, call goes through
mv88e6xxx_port_vlan_prepare() that calls mv88e6xxx_port_check_hw_vlan()
that returns -EOPNOTSUPP for zero vid.

This patch changes mv88e6xxx_vtu_get() to also return -EOPNOTSUPP for
zero vid, then this error code is explicitly cleared in
dsa_slave_vlan_rx_kill_vid() and error message is no longer logged.
Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
Reviewed-by: Vivien Didelot <vivien.didelot@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>