[TG3]: Fix TSO bugs.

1. Remove the check for skb->len greater than MTU when doing TSO.
When the destination has a smaller MSS than the source, a TSO packet
may be smaller than the MTU and we still need to process it as a TSO
packet.

2. On 5705A3 devices with TSO enabled, the DMA engine can hang due to a
hardware bug.  This patch avoids the hanging condition by reducing the
DMA burst size.

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