net: dsa: make the .flow_dissect tagger callback return void

There is no tagger that returns anything other than zero, so just change
the return type appropriately.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>