net: dsa: use phy_ethtool_nway_reset

Use phy_ethtool_nway_reset now that dsa_slave_nway_reset does exactly
the same.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>