[FIX] pos_loyalty: specific discountable take discount into account

Current behavior:
When computing the discountable values of an order, the discount of the
program where not taken into account if you used specific discountable
products.

Steps to reproduce:
- Create products A and B with a price of 20$
- Create a loyalty program
- Rule:
  - Minimum qty : 2
  - Apply on specific products : Product A and Product B
  - Reward : 5 points per order
- Reward:
  - 10$ per order
  - In exchange of 2 points
  - Apply on specific products : Product A and Product B
- Open PoS and add Product A and Product B to the order
- A reward should be added automatically with a value of -10$
- Click on the reward button, another reward should be added but it has
  the wrong value. It should be -10$ but it is not.

opw-3232565

closes odoo/odoo#121209

Signed-off-by: Joseph Caburnay (jcb) <jcb@odoo.com>