dsa: slave: eee: Allow ports to use phylink

For a port to be able to use EEE, both the MAC and the PHY must
support EEE. A phy can be provided by both a phydev or phylink. Verify
at least one of these exist, not just phydev.

Fixes: aab9c406

 ("net: dsa: Plug in PHYLINK support")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>