e1000e: Additional PHY power saving in S0ix

After transferring the MAC-PHY interface to the SMBus set the PHY
to S0ix low power idle mode.

Suggested-by: Dima Ruinskiy <dima.ruinskiy@intel.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>