dsa: mv88e6xxx: Use bus in mv88e6xxx_lookup_name()

mv88e6xxx_lookup_name() returns the model name of a switch at a given
address on an MII bus. Using mii_bus to identify the bus rather than
the host device is more logical, so change the parameter.

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