r8169: ensure tx_skb is fully reset after calling rtl8169_unmap_tx_skb

So far tx_skb->skb is the only member of the two structs that is not
reset. Make understanding the code easier by resetting both structs
completely in rtl8169_unmap_tx_skb.

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