purchase double validation limit is not kept after update

Impacted versions:

 - 7.0

Steps to reproduce:

1. In "Settings>Configuration>Purchase" change "the limit to require a second approval" (with 1000 instead of 5000) and apply
2. In "Settings>Modules>Installed Modules" update module "purchase_double_validation"

Current behavior:

 - The limit is 5000

Expected behavior:

 - The limit should be 1000