r8169: let rtl8169_mark_to_asic clear rx descriptor field opts2

Clearing opts2 belongs to preparing the descriptor for DMA engine use.
Therefore move it into rtl8169_mark_to_asic().

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