tg3: Cleanup IPV6 LSO

This patch attempts to make the relationship between IPV6 checksum
offload and IPV6 LSO more obvious.  The patch also toggles a bit
needed for IPV6 LSO on 5785 and 57780 devices.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>