Observations of VLANs dropping packets due to invalid
checksums when not offloading VLAN tag receive.
With VLAN tag stripping enabled no issue is observed.

Drop back to s/w checksums if VLAN offload is disabled.
Signed-off-by: NDave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>