ethtool: Core impl for ETHTOOL_PHY_DOWNSHIFT tunable

Adding validation support for the ETHTOOL_PHY_DOWNSHIFT. Functional
implementation needs to be done in the individual PHY drivers.

Signed-off-by: Raju Lakkaraju <Raju.Lakkaraju@microsemi.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Allan W. Nielsen <allan.nielsen@microsemi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>