[FIX] mrp_repair: cannot create a repair if not access to product.product

Steps to reproduce the bug:

- Connect as demo user
- Create a repair
- Add a product
- Save

Bug: An access error was raised because the demo user has no access rights
to write on product.product

opw:816645

addons/mrp_repair/views/mrp_repair_views.xml

@@ -50,7 +50,7 @@
                     </h1>
                     <group>
                         <group>
-                            <field name="tracking" invisible="1"/>
+                            <field name="tracking" invisible="1" attrs="{'readonly': 1}"/>
                             <field name="product_id" domain="[('type', 'in', ['product', 'consu'])]"/>
                             <label for="product_qty"/>
                             <div class="o_row">