ravb: remove custom .set_link_ksettings from ethtool ops

The generic phy_ethtool_set_link_ksettings() function from phylib can
be used instead of in-house ravb_set_link_ksettings().

Signed-off-by: Vladimir Zapolskiy <vladimir_zapolskiy@mentor.com>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>