net: dsa: mv88e6xxx: Return -ENODEV when C45 not supported

MDIO bus drivers can return -ENODEV when they know the bus does not
have a device at the given address, e.g. because of hardware
limitation. One such limitation is that the bus does not support C45
at all. This is more efficient than returning 0xffff, since it
immediately stops the probing on the given address, where as further
reads can be made when 0xffff is returned.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>