ftgmac100: Add ethtool n-way reset call

A non-wired up implementation accidentally made its way in
a previous patch (Make ring sizes configurable via ethtool).

This removes it and wires up the generic phy_ethtool_nway_reset
instead.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>