r8169: make use of xmit_more and __netdev_sent_queue

Make use of xmit_more and add the functionality introduced with
3e59020a

 ("net: bql: add __netdev_tx_sent_queue()").
I used the mlx4 driver as template.

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