r8169: split rtl8169_tso_csum

According to the txd_version, split rtl8169_tso_csum() into
rtl8169_tso_csum_v1() and rtl8169_tso_csum_v2().

Signed-off-by: Hayes Wang <hayeswang@realtek.com>
Signed-off-by: David S. Miller <davem@davemloft.net>