[donation] fix validation for select_or_other

- problem: the validation does never succeed when select_or_other is
  enabled on donation_amount
- reason: $element['#value'] is not set by select_or_other, the value is
  store in the $form_state
- also needed to add select_or_other_element_validate manually, as this
  is not going to be set by select_or_other if an #element_validate
  already exists