net: dsa: mv88e6xxx: add port ATU learn limit op

Add a new operation to disable the limiting of learnt MAC addresses.

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

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>