tg3: Fix 5906 transmit hangs

The 5906 has trouble with fragments that are less than 8 bytes in size.
This patch works around the problem by pivoting the 5906's transmit
routine to tg3_start_xmit_dma_bug() and introducing a new SHORT_DMA_BUG
flag that enables code to detect and react to the problematic condition.

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>