sch_htb: fix doc warning in htb_do_events()

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

net/sched/sch_htb.c:708: warning: Function parameter or member 'q' not described in 'htb_do_events'
net/sched/sch_htb.c:708: warning: Function parameter or member 'level' not described in 'htb_do_events'
net/sched/sch_htb.c:708: warning: Function parameter or member 'start' not described in 'htb_do_events'

Signed-off-by: Yu Kuai <yukuai3@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>