tg3: Reintroduce tg3_tx_ring_info

The following patches will require the use of an additional flag in the
ring_info structure.  The use of this flag is tx path specific, so this
patch defines a specialized ring_info structure.

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