net: dsa: allow use of phylink managed EEE support

net: dsa: allow use of phylink managed EEE support

In order to allow DSA drivers to use phylink managed EEE, we need to
change the behaviour of the DSA's .set_eee() ethtool method.
Implementation of the DSA .set_mac_eee() method becomes optional with
phylink managed EEE as it is only used to validate the EEE parameters
supplied from userspace. The rest of the EEE state management should
be left to phylink.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1thR9l-003vXC-9F@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>