net: dsa: mv88e6xxx: Only support EDSA tagging

The merged driver no longer offers the option to use DSA tagging. So
remove the code to setup the switch to do DSA tagging and hard code
the use of EDSA.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com&gt;y>
Signed-off-by: David S. Miller <davem@davemloft.net>