r8169: remove jumbo_tx_csum from chip config struct

According to the chip configuration entries only RTL8169 (ver <= 06)
supports tx checksumming for jumbo packets.
By the way: constant JUMBO_1K is a little misleading because it refers
to the standard packet size and not to a jumbo packet size.

By implementing this rule we can get rid of configuring tx checksumming
support per chip type.

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