net: microchip: sparx5: Ensure L3 protocol has a default value

This ensures that the l3_proto always have a valid value and that any
dissector parsing errors causes the flower rule to be discarded.

Signed-off-by: Steen Hegelund <steen.hegelund@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>