r8169: enable ASPM L0s state

ASPM is disabled completely because we've seen different types of
problems in the past. However it seems these problems occurred with
L1 or L1 sub-states only. On all the chip versions I've seen the
acceptable L0s exit latency is 512ns. This should be short enough not
to cause problems. If the actual L0s exit latency of the PCIe link
is bigger than 512ns then the PCI core will disable L0s anyway.
So let's give it a try and disable L1 and L1 sub-states only.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>