tg3: Assign flags to fixes in start_xmit_dma_bug

This patch adds a flag for each bug workaround in
tg3_start_xmit_dma_bug().  This is prep work for the following patch.

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