cassini: Only use chip checksum for ipv4 packets.

According to David Monro, at least with Natsemi Saturn chips the
cassini driver has some trouble with ipv6 checksums.

Until we have more information about what's going on here, only
use the chip checksums for ipv4.

This workaround was suggested and tested by David.

Update version and release date.

Signed-off-by: David S. Miller <davem@davemloft.net>