tg3: Use skb_is_gso_v6()

This patch converts the driver to prefer the skb_is_gso_v6() helper over
the explicit inlined version.

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