r8169: enable ASPM states via sysfs

[Heiner Kallweit]

So far ASPM is disabled in the r8169 driver due to issues on several
chip version / BIOS version combinations. Commit ad46fe1c7336
("PCI/ASPM: Add sysfs attributes for controlling ASPM link states")
adds the option to enable ASPM states for a device via sysfs
(provided that BIOS allows the OS to control ASPM).
This commit is included in latest linux-next and may be useful for
notebook users to benefit from ASPM power saving.

Heiner