sunvnet: restrict advertized checksum offloads to just IP

As much as we'd like to play well with others, we really aren't
handling the checksums on non-IP protocol packets very well.  This
is easily seen when trying to do TCP over ipv6 - the checksums are
garbage.

Here we restrict the checksum feature flag to just IP traffic so
that we aren't given work we can't yet do.

Orabug: 26175391, 26259755

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>