net: dsa: PHY device is mandatory for EEE

The port's PHY and MAC are both implied in EEE. The current code does
not call the PHY operations if the related device is NULL. Change that
by returning -ENODEV if there's no PHY device attached to the interface.

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>