[FIX] stock: Not allowed to change product on stock.production.lot

Steps to reproduce the bug:
-Create a product tracked by LN
-Create an inventory adjustment to add 10 products with LN A
-Validate it
-Then go in inventory > inventory control > lots/serial numbers > open LN A
-Change the product

Bug:
The LN A tells you you have 10 products X
While in your inventory you see you have 10 products Y

opw:750735

addons/stock/views/stock_production_lot_views.xml

@@ -24,7 +24,7 @@
                 </div>
                 <group name="main_group">
                     <group>
-                        <field name="product_id"/>
+                        <field name="product_id" attrs="{'readonly':[('quant_ids','!=',[])]}"/>
                             <label for="product_qty"/>
                             <div class="o_row">
                                 <field name="product_qty"/>