r8169: implement callback ndo_features_check

Implement callback ndo_features_check and move all feature checks there.
This will allow us to get rid of r8169_csum_workaround() completely in
a subsequent step. Like in the vendor driver disable HW csum for short
packets on RTL8168b.

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