r8169: add rtl_disable_exit_l1()

Add rtl_disable_exit_l1() for ensuring that the chip doesn't
inadvertently exit ASPM L1 when being in a low-power mode.
The new function is called from rtl_prepare_power_down() which
has to be moved in the code to avoid a forward declaration.

According to Realtek OCP register 0xc0ac shadows ERI register 0xd4
on RTL8168 versions from RTL8168g. This allows to simplify the
code a little.

v2:
- call rtl_disable_exit_l1() also if DASH or WoL are enabled

Suggested-by: Chun-Hao Lin <hau@realtek.com>
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>