dsa: mv88e6xxx: Set DSA mode based on chip abilities

Older devices only support a single DSA frame format, where as newer
devices have two. Take this into account when configuring a DSA port.
The port needs to be in plain old DSA mode, since this is a DSA link,
where as the newer format can be used for the CPU port.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>