r8169: inline rtl8169_free_rx_databuff

rtl8169_free_rx_databuff is used in only one place, so let's inline it.
We can improve the loop because rtl8169_init_ring zero's RX_databuff
before calling rtl8169_rx_fill, and rtl8169_rx_fill fills
Rx_databuff starting from index 0.

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