r8169: drop member txd_version from struct rtl8169_private

txd_version is used in rtl_init_one() only, so we can drop member
txd_version from struct rtl8169_private.

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