net: dsa: mv88e6xxx: Fix support for DSA tagging for older switches.

Older chips only support DSA tagging on the CPU port. New devices
support both DSA and EDSA. The driver needs to tell the core the tag
protocol to use, and configure the switch for what is available.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>