tg3: Fix 4k tx bd segmentation code

The new 4k tx bd segmentation code had a bug in the error cleanup path.
If the driver did not map all the physical fragments, the abort path
would wind up advancing the producer index beyond the point where the
setup code stopped.  This would ultimately turn into a tx recovery error
where the driver would expect the skb pointer to be set when it isn't.
This patch fixes the problem, and then makes the code a little easier to
understand.

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