r8169: inline rtl8169_make_unusable_by_asic

Inline rtl8169_make_unusable_by_asic() and simplify it:
- Address field doesn't need to be poisoned because descriptor is
  owned by CPU now
- desc->opts1 is set by rtl8169_mark_to_asic() and rtl8169_rx_fill(),
  therefore we don't have to preserve any field parts.

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