net: phy: add phy_ethtool_{get|set}_link_ksettings

Ethtool callbacks {get|set}_link_ksettings are often the same, so
we add two generics functions phy_ethtool_{get|set}_link_ksettings
to avoid writing severals times the same function.

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