mlx4: fix error path in drivers/net/mlx4/en_rx.c

this warning:

  drivers/net/mlx4/en_rx.c: In function ‘mlx4_en_activate_rx_rings’:
  drivers/net/mlx4/en_rx.c:412: warning: ‘err’ may be used uninitialized in this function

Triggers because 'err' is uninitialized in the following input
conditions: priv->rx_ring_num is zero and mlx4_en_fill_rx_buffers()
fails.

But even if ->rx_ring_num is nonzero, 'err' will be zero if
mlx4_en_fill_rx_buffers() fails and mlx4_en_activate_rx_rings() returns
success - incorrectly.

So it's best to keep the error code uptodate on mlx4_en_fill_rx_buffers()
calls as well.
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/mlx4/en_rx.c

@@ -443,7 +443,8 @@ int mlx4_en_activate_rx_rings(struct mlx4_en_priv *priv)
 		/* Fill Rx buffers */
 		ring->full = 0;
 	}
-	if (mlx4_en_fill_rx_buffers(priv))
+	err = mlx4_en_fill_rx_buffers(priv);
+	if (err)
 		goto err_buffers;
 
 	for (ring_ind = 0; ring_ind < priv->rx_ring_num; ring_ind++) {