tg3: Add new HW_TSO_3 flag for 5717

The 5717 sets up TSO slightly differently in the transmit path.  It
looks like this method will be the new way of doing things.  This patch
defines a flag to indicate this.

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