net: dsa: mv88e6131: use EDSA tag protocol

6131 is the only driver to set the tag protocol to DSA_TAG_PROTO_DSA.
Since it works fine with DSA_TAG_PROTO_EDSA, change its value, like all
other mv88e6xxx drivers.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>