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>