net: dsa: tag_dsa: use the generic flow dissector procedure

Remove the .flow_dissect procedure, so the flow dissector will call the
generic variant which works for this tagging protocol.

Cc: Andrew Lunn <andrew@lunn.ch>
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>