r8169: improve handling of TD_MSS_MAX

If the mtu is greater than TD_MSS_MAX, then TSO is disabled, see
rtl8169_fix_features(). Because mss is less than mtu, we can't have
the case mss > TD_MSS_MAX in the TSO path.

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