lib: updatehub: Add missing do upgrade request call

After a success image download, UpdateHub needs inform MCUboot that
must test new image and then, on success, commit this new image. This
add missing upgrade request call step and fixes the upgarde flow.

Signed-off-by: Gerson Fernando Budke <gerson.budke@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
(cherry picked from commit d1e2d345)