ravb: remove custom .nway_reset from ethtool ops

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

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>