portability: cmsis: Fix possible race in osEventFlagsClear()

The CMSIS-RTOS specification says "The function returns the event flags
before clearing".

In the original code, if another thread set an event between k_event_test()
and k_event_clear(), there was a risk the function clear a flag without
reporting it to the caller:

        T1                                   T2
  k_event_test(..) == 0                      ...
        ...                             k_event_post(.. 1)
  k_event_clear(.. 1)                        ...
  return 0 (while event 1 has been cleared)

Signed-off-by: Jérôme Pouiller <[email protected]>

Author: jerome-pouiller

subsys/portability/cmsis_rtos_v2/event_flags.c

@@ -79,7 +79,7 @@ uint32_t osEventFlagsClear(osEventFlagsId_t ef_id, uint32_t flags)
 	}
 
 	rv = k_event_test(&events->z_event, 0xFFFFFFFF);
-	k_event_clear(&events->z_event, flags);
+	k_event_clear(&events->z_event, flags & rv);
 
 	return rv;
 }