net: dsa: mv88e6xxx: remove EEE support

The PHY's EEE settings are already accessed by the DSA layer through the
Marvell PHY driver and there is nothing to be done for switch's MACs.

Remove all EEE support from the mv88e6xxx driver and simply return 0
from the EEE ops.

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>