pm: runtime: fix race when waiting for suspended event

To wait for the asynchronous suspending work item to complete, a
combination of semaphores and events is used. First, the semaphore is
released, then the events are cleared (through the boolean argument to
k_event_wait), then events are awaited.

However, if the event flag happens to be set by the work handler in the
short time between k_sem_give and k_event_wait, it is then cleared by
k_event_wait and k_event_wait blocks forever waiting for the event.

Make sure that we clear the event flag before releasing the semaphore.

Signed-off-by: Marco Widmer <marco.widmer@bytesatwork.ch>