r8169: don't use bit LastFrag in tx descriptor after send

On RTL8125 this bit is always cleared after send. Therefore check for
tx_skb->skb being set what is functionally equivalent.

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