When setting the ethtool settings, ensure that the validated PHY
interface mode is propagated to the current link settings, so that
2500BaseX can be selected.

Fixes: 9525ae83 ("phylink: add phylink infrastructure")
Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>