cpsw: ethtool: add support for nway reset

This patch adds support for ethtool's '-r' command. Restarting
N-WAY negotiation can be useful to activate newly changed EEE
settings etc.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>