Add a new operation to disable the DA, SA and VTU priority override.

Setting such limit is not likely to be used soon, so provide a
port_disable_pri_override operation directly. This can be changed later
for port_set_pri_override when we'll need it.

Also remove the now obsolete mv88e6xxx_6320_family helper.
Signed-off-by: NVivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: NAndrew Lunn <andrew@lunn.ch>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>