GRETH: avoid overwrite IP-stack's IP-frags checksum

The GRETH GBIT core does not do checksum offloading for IP
segmentation. This patch adds a check in the xmit function to
determine if the stack has calculated the checksum for us.

Signed-off-by: Daniel Hellstrom <daniel@gaisler.com>
Signed-off-by: David S. Miller <davem@davemloft.net>