kernel: coding guidelines: add explicit cast to void

- added explicit cast to void when returned value is expectedly ignored

Signed-off-by: frei tycho <tfrei@baumer.com>

kernel/smp.c

@@ -74,7 +74,7 @@ void z_smp_global_unlock(unsigned int key)
 		_current->base.global_lock_count--;
 
 		if (!_current->base.global_lock_count) {
-			atomic_clear(&global_lock);
+			(void)atomic_clear(&global_lock);
 		}
 	}
 
@@ -85,7 +85,7 @@ void z_smp_global_unlock(unsigned int key)
 void z_smp_release_global_lock(struct k_thread *thread)
 {
 	if (!thread->base.global_lock_count) {
-		atomic_clear(&global_lock);
+		(void)atomic_clear(&global_lock);
 	}
 }