r8169: fix driver shutdown WoL regression.

Due to commit 92fc43b4

 ("r8169: modify the
flow of the hw reset."), rtl8169_hw_reset stomps during driver shutdown on
RxConfig bits which are needed for WOL on some versions of the hardware.

As these bits were formerly set from the r81{0x, 68}_pll_power_down methods,
factor them out for use in the driver shutdown (rtl_shutdown) handler.

I favored __rtl8169_get_wol() -hardware state indication- over
RTL_FEATURE_WOL as the latter has become a good candidate for removal.

Signed-off-by: Francois Romieu <romieu@fr.zoreil.com>
Cc: Hayes <hayeswang@realtek.com>
Tested-by: Marc Ballarin <ballarin.marc@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>