drivers/net: starfire: Fix napi ->poll() weight handling

starfire napi ->poll() handler can return work == weight after calling
netif_rx_complete() (if there is no more work). It is illegal and this
patch fixes it.

Reported-by: Alexander Huemer <alexander.huemer@sbg.ac.at>
Tested-by: Alexander Huemer <alexander.huemer@sbg.ac.at>
Signed-off-by: Jarek Poplawski <jarkao2@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>