how to limit the rotation of an object by tilting the device?

I'm doing with the plan accompanying the slope of a device, it happens that when I lean up to 90 degrees the plan continues to lean against a rotation up to 360 degrees. How to limit?

see how is my script.

 using UnityEngine;
 using System.Collections;
 
 public class SceneScripts : MonoBehaviour {
 
     public Vector3 rotateDir = Vector3.zero;
     public float rotateSpeed = 3.0F;
 
     // Update is called once per frame
     void Update() {
 
         rotateDir.x = -Input.acceleration.y;
         rotateDir.z = -Input.acceleration.x;
 
         if (rotateDir.sqrMagnitude > 0.2)
             rotateDir.Normalize();
 
         transform.Rotate(rotateDir * rotateSpeed);
     }
 }