net: dsa: ksz: implement .support_eee() method

Implement the .support_eee() method by reusing the ksz_validate_eee()
method as a template, renaming the function, changing the return type
and values, and removing it from the ksz_set_mac_eee() and
ksz_get_mac_eee() methods.

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/E1tL14Z-006cZs-6o@rmk-PC.armlinux.org.uk


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