net: dsa: mv88e6xxx: add a cascade port op

Only the 88E6185 family has bits 15:12 Cascade Port bits in the Global
Control 2 register. Hence inconsistent values are actually written in
this register for other families.

Add a .set_cascade_port operation to isolate the 88E6185 case, and call
it from the device mapping setup function.

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>