net: dsa: mv88e6xxx: use already declared variables

In the MDIO probing function, dev is already assigned to &mdiodev->dev
and np is already assigned to mdiodev->dev.of_node, so use them.

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>