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: Ingo Molnar <mingo@elte.hu>
Signed-off-by: David S. Miller <davem@davemloft.net>