net: dsa: mv88e6xxx: Avoid useless attempts to fast-age LAGs

When a port is a part of a LAG, the ATU will create dynamic entries
belonging to the LAG ID when learning is enabled. So trying to
fast-age those out using the constituent port will have no
effect. Unfortunately the hardware does not support move operations on
LAGs so there is no obvious way to transform the request to target the
LAG instead.

Instead we document this known limitation and at least avoid wasting
any time on it.

Signed-off-by: Tobias Waldekranz <tobias@waldekranz.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>