pm_layered: Use {} for empty while loops

sys/pm_layered/pm.c

@@ -88,10 +88,10 @@ void pm_unblock(unsigned mode)
 }
 
 #ifndef PROVIDES_PM_LAYERED_OFF
-void  pm_off(void)
+void pm_off(void)
 {
     pm_blocker.val_u32 = 0;
     pm_set_lowest();
-    while(1);
+    while(1) {}
 }
 #endif