net: dsa: ksz: Select KSZ protocol tag

KSZ protocol tag is needed by the KSZ DSA drivers.

Fixes: 0b9f9dfb

 ("dsa: Allow tag drivers to be built as modules")
Tested-by: Cristian Birsan <cristian.birsan@microchip.com>
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>