e1000e: Fix EEE in Sx implementation

This patch implements the EEE in Sx code so that it only applies to parts
that support EEE in Sx (as opposed to all parts that support EEE).
It also uses the existing eee_advert and eee_lp_abiliity to set just the
bits (100/1000) that should be set.

Signed-off-by: Raanan Avargil <raanan.avargil@intel.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>