net: dsa: return -ENODEV is there is no slave PHY

Instead of returning -EOPNOTSUPP when a slave device has no PHY,
directly return -ENODEV as ethtool and phylib do.

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