drivers: dm8806: dm8806: disable ethernet efficient mode

Energy efficient mode is the feature of the DM8806 described in the
EEE 802.3az Energy Efficient for reducing power consumption. For unknown
reason it seams to not working correctly with all endpoints and sometimes
the network randomly fails when this mode is on. Thus it is now possible to
turn off it in compile time by KConfig option:
PHY_DM8806_ENERGY_EFFICIENT_MODE if in case of network problems

Signed-off-by: Robert Slawinski <robert.slawinski.ext@indurad.com>