sch_htb: fix doc warning in htb_next_rb_node()

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

net/sched/sch_htb.c:339: warning: Function parameter or member 'n' not described in 'htb_next_rb_node'

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