sch_htb: fix doc warning in htb_lookup_leaf()

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

net/sched/sch_htb.c:773: warning: Function parameter or member 'hprio' not described in 'htb_lookup_leaf'
net/sched/sch_htb.c:773: warning: Function parameter or member 'prio' not described in 'htb_lookup_leaf'

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