phylink: ensure the PHY interface mode is appropriately set

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>