[TG3]: Fix 4GB boundary tx handling

Fix and simplify the workaround code for the 4GB boundary tx buffer
hardware bug.

1. Need to unmap the original SKB's dma addresses if a new SKB cannot
   be allocated.

2. Need to pass the base flag to tigon3_4gb_hwbug_workaround() or TSO
   won't work properly.

3. The guilty entry and length parameters for
   tigon3_4gb_hwbug_workaround() are removed as they are not necessary.

4. Remove assumption that only one fragment can hit the 4GB boundary.
   Another fragment can hit 8GB for example.

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