Return used line item instead the order in commerce_cart_product_add()

I think it's more useful to return the used , probably new, line item object instead the order in commerce_cart_product_add().
This since it's easier to load the appropriate order object afterwards than iterating over all line item objects to find the newly added one.
I quickly searched in the code and it looks like the return of the function is not used anywhere yet. Thus the change shouldn't have a big impact.