tg3: Fix IPv6 TSO code in tg3_start_xmit_dma_bug()

The tg3_start_xmit_dma_bug() function was missing code to process IPv6
TSO packets.  This patch adds the missing support.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>