net: dsa: provide implementation of .support_eee()

Provide a trivial implementation for the .support_eee() method which
switch drivers can use to simply indicate that they support EEE on
all their user ports.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Link: https://patch.msgid.link/E1tL149-006cZJ-JJ@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>