r8169: rename rtl_csi_access_enable to rtl_set_aspm_entry_latency

Rename the function to reflect what it's doing. Also add a description
of the register values as kindly provided by Realtek.

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