Auth Provider Form AJAX doesn't change the form after the first time

The Auth Provider drop down on /admin/config/salesforce/authorize/add doesn't actually change the auth fields after you select a provider once.

Tiny patch that:

• Checks if the form is rebuilding and if it is, check the provider value again rather than using the already loaded plugin.
• Removes an extra condition for default, the else on line 85 is redundant since $form['settings']['provider_settings'] is always set to default via line 76.