net: dsa: Provide unique DSA slave MII bus names

In case we have multiples trees and switches with the same index, we
need to add another discriminating id: the switch tree.

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