e1000e: Export S0ix flags to ethtool

This flag can be used by an end user to disable S0ix flows on a
buggy system or by an OEM for development purposes.

If you need this flag to be persisted across reboots, it's suggested
to use a udev rule to call adjust it until the kernel could have your
configuration in a disallow list.

Signed-off-by: Mario Limonciello <mario.limonciello@dell.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Tested-by: Yijun Shen <Yijun.shen@dell.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>