mlx4_core: Fix one more static exported function

Commit 22c8bff6 ("mlx4_core: Exported functions can't be static")
fixed most of this up, but forgot about mlx4_is_slave_active().  Fix
this one too.

Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>

drivers/net/ethernet/mellanox/mlx4/main.c

@@ -394,7 +394,7 @@ static int mlx4_how_many_lives_vf(struct mlx4_dev *dev)
 	return ret;
 }
 
-static int mlx4_is_slave_active(struct mlx4_dev *dev, int slave)
+int mlx4_is_slave_active(struct mlx4_dev *dev, int slave)
 {
 	struct mlx4_priv *priv = mlx4_priv(dev);
 	struct mlx4_slave_state *s_slave;