net: dsa: mv88e6xxx: remove redundant assignments

The chip->ds and ds->slave_mii_bus assignments are common to both legacy
and new MDIO probing and are already done in the later setup code.

Remove the duplicated assignments from the MDIO probing 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>