net: thunderx: Allow IPv6 frames with zero UDP checksum

Do not consider IPv6 frames with zero UDP checksum as frames
with bad checksum and drop them.

Signed-off-by: Thanneeru Srinivasulu <tsrinivasulu@cavium.com>
Signed-off-by: Sunil Goutham <sgoutham@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>