[BNX2]: Refine napi poll loop.

Need to read and store sblk->status_idx before checking for more work.
The status idx 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 completing bnx2_poll(), we should always break out of the while
loop and return work_done instead of returning 0.

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