net: dsa: mv88e6xxx: set ethtool regs version

Currently the ethtool_regs version is set to 0 for all DSA drivers.

Use this field to store the chip ID to simplify the pretty dump of
any interfaces registered by the "dsa" driver.

Signed-off-by: Vivien Didelot <vivien.didelot@gmail.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>