ethtool: Implements ETHTOOL_PHY_GTUNABLE/ETHTOOL_PHY_STUNABLE

Adding get_tunable/set_tunable function pointer to the phy_driver
structure, and uses these function pointers to implement the
ETHTOOL_PHY_GTUNABLE/ETHTOOL_PHY_STUNABLE ioctls.

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>