net: dsa: use phy_ethtool_set_link_ksettings

Use phy_ethtool_set_link_ksettings now that dsa_slave_set_link_ksettings
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>