r8169: improve saved_wolopts handling

Let's make saved_wolopts a shadow copy of the WoL options. This allows
to simplify the code and get rid of calls to now unneeded function
__rtl8169_get_wol(). However don't remove __rtl8169_get_wol()
completely to be prepared for the case that we can respect BIOS WOL
settings again.

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