net: lan78xx: Request s/w csum check on VLAN tagged packets.

There appears to be some issue in the LAN78xx where the checksum
computed on a VLAN tagged packet is incorrect, or at least not
in the form that the kernel is after. This is most easily shown
by pinging a device via a VLAN tagged interface and it will dump
out the error message and stack trace from netdev_rx_csum_fault.
It has also been seen with standard TCP and UDP packets.

Until this is fully understood, request that the network stack
computes the checksum on packets signalled as having a VLAN tag
applied.

See https://github.com/raspberrypi/linux/issues/2458



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>