net: lan78xx: Use s/w csum check on VLANs without tag stripping

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: Dave Stevenson <dave.stevenson@raspberrypi.org>
Signed-off-by: David S. Miller <davem@davemloft.net>