mgmt: hawkbit: also clear in the call of hawkbit_autohandler()

the k_event_clear in the work might be to late sometimes, when
hawkbit_autohandler_wait() is executed directly after
hawkbit_autohandler(). This leads to getting the events
of the former execution and also not waiting until the
current autohandler run is finished.

Signed-off-by: Fin Maaß <f.maass@vogl-electronic.com>
(cherry picked from commit e59451a7)