sch_htb: fix doc warning in htb_class_mode()

Add description for parameters of htb_class_mode()
to fix gcc W=1 warnings:

net/sched/sch_htb.c:507: warning: Function parameter or member 'cl' not described in 'htb_class_mode'
net/sched/sch_htb.c:507: warning: Function parameter or member 'diff' not described in 'htb_class_mode'
Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_htb.c

@@ -510,6 +510,8 @@ static inline s64 htb_hiwater(const struct htb_class *cl)
 
 /**
  * htb_class_mode - computes and returns current class mode
+ * @cl: the target class
+ * @diff: diff time in microseconds
  *
  * It computes cl's mode at time cl->t_c+diff and returns it. If mode
  * is not HTB_CAN_SEND then cl->pq_key is updated to time difference