net: dsa: mv88e6xxx: set device number

All Marvell switches supported by mv88e6xxx have to set their device
number in the Global Control 2 register. Extract this in a read then
write function, called from the device mapping setup code.

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