[PKT_SCHED]: Fix sch_prio.c build with CONFIG_NETDEVICES_MULTIQUEUE

Fix one more user of netiff_subqueue_stopped. To check for the
queue id one must use the __netiff_subqueue_stoped call.

This run out of my sight when I made the:

668f895a
[NET]: Hide the queue_mapping field inside netif_subqueue_stopped

commit :(

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>