niu: panic on reset

The reset_task function in the niu driver does not reset the tx and rx
buffers properly. This leads to panic on reset. This patch is a
modified implementation of the previously posted fix.

Signed-off-by: Santwona Behera <santwona.behera@sun.com>
Signed-off-by: David S. Miller <davem@davemloft.net>