r8169: improve rtl8169_rx_csum

Extend the mask to include the checksum failure bits. This allows to
simplify the condition in rtl8169_rx_csum().

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Saeed Mahameed <saeedm@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>