[TG3]: Refine napi poll loop.

Need to read and store sblk->status_tag before checking for more work.
The status tag is later written back to the hardware when enabling
interrupts to acknowledge how much work has been processed.  If the
order is reversed, we can end up acknowledging work we haven't
processed.

When we detect tx error, it is more correct to return the rx
work_done so far instead of 0.

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