ar8216: introduce enable_eee swconfig attribute to control 802.3az EEE per port

ar8216: introduce enable_eee swconfig attribute to control 802.3az EEE per port

Users reported network issues with AR8327 which turned out to be caused
by EEE not working correctly with certain link partners (ticket 14597).
The workaround was to disable EEE on all ports (changeset 41577).

The issue was with certain link partners only, therefore this patch
allows to control usage of EEE per port via swconfig.
Still the default is to initially disable EEE on all ports.

Successfully tested on a TL-WDR4900 (AR8327 rev.4)

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44021 3c298f89-4303-0410-b956-a3cf2f4a3e73