net: dsa: Centralise getting switch id

Get the switch id and save it away in the private mv88x6xxx structure
in a centralised piece of code, rather than each driver doing it itself.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: David S. Miller <davem@davemloft.net>