phy: add generic function to support ksetting support

The old ethtool api (get_setting and set_setting) has
generic phy functions phy_ethtool_sset and phy_ethtool_gset.
To supprt the new ethtool api (get_link_ksettings and
set_link_ksettings), we add generic phy function
phy_ethtool_ksettings_get and phy_ethtool_ksettings_set.

Signed-off-by: Philippe Reynes <tremyfr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>