mlxsw: spectrum_switchdev: Flush the mdb when a port is being removed

When a port is being removed from a bridge, flush the bridge mdb to remove
the mids of that port.

Signed-off-by: Nogah Frankel <nogahf@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>