tg3: Add New 5719 Read DMA workaround

tg3: Add New 5719 Read DMA workaround

After Power-on-reset, the 5719's TX DMA length registers may contain
uninitialized values and cause TX DMA to stall.  Check for invalid
values and set a register bit to flush the TX channels.  The bit
needs to be turned off after the DMA channels have been flushed.

(cherry picked from commit 091f0ea30074bc43f9250961b3247af713024bc6)
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Joe Jin <joe.jin@oracle.com>