switchdev: Add support for flood control

Allow devices supporting this feature to control the flooding of unknown
unicast traffic, by making switchdev infrastructure propagate this setting
to the switch driver.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>