r8169: simplify usage of rtl8169_unmap_tx_skb

Simplify the parameters taken by rtl8169_unmap_tx_skb, this makes
usage of this function easier to read and understand.

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