net: dsa: mv88e6xxx: Offload bridge learning flag

Allow a user to control automatic learning per port.

Many chips have an explicit "LearningDisable"-bit that can be used for
this, but we opt for setting/clearing the PAV instead, as it works on
all devices at least as far back as 6083.

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>