[TG3]: Add new hard_start_xmit

Support 5787 hardware TSO using a new flag TG3_FLG2_HW_TSO_2.

Since the TSO interface is slightly different and these chips have
finally fixed the 4GB DMA problem and do not have the 40-bit DMA
problem, a new hard_start_xmit is used for these chips. All previous
chips will use the old hard_start_xmit that is now renamed
tg3_start_xmit_dma_bug().

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>