[FIX] mrp: change production quantity is wrong on finished moves

Usecase to reproduce:
- BoM -> 1 component for 1 final product
- Create a MO with a quanity of 5
- Update the quantity to 6

-> The move finished has an initial demand of 4

addons/mrp/tests/test_order.py

@@ -516,6 +516,7 @@ class TestMrpOrder(TestMrpCommon):
         update_quantity_wizard.change_prod_qty()
 
         self.assertEqual(mo.move_raw_ids.filtered(lambda m: m.product_id == p1).quantity_done, 20, 'Update the produce quantity should not impact already produced quantity.')
+        self.assertEqual(mo.move_finished_ids.product_uom_qty, 4)
         mo.button_mark_done()
 
     def test_product_produce_6(self):

addons/mrp/wizard/change_production_qty.py

@@ -35,8 +35,8 @@ class ChangeProductionQty(models.TransientModel):
         modification = {}
         for move in production.move_finished_ids.filtered(lambda m: m.state not in ('done', 'cancel')):
             qty = (qty - old_qty) * move.unit_factor
-            modification[move] = (move.product_uom_qty - qty, move.product_uom_qty)
-            move[0].write({'product_uom_qty': move.product_uom_qty - qty})
+            modification[move] = (move.product_uom_qty + qty, move.product_uom_qty)
+            move[0].write({'product_uom_qty': move.product_uom_qty + qty})
         return modification
 
     def change_prod_qty(self):