ravb: remove custom .get_link_ksettings from ethtool ops

The generic phy_ethtool_get_link_ksettings() function from phylib can be
used instead of in-house ravb_get_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>