examples/qos_sched: fix dequeue from ring

examples/qos_sched: fix dequeue from ring

The app_worker_thread() and app_mixed_thread() use rte_ring_sc_dequeue_bulk
to dequeue packets from the ring and this imposes restriction on number of
packets in software ring to be greater than the specified value to start
actual dequeue operation, thus, adds latency to those packets. Therefore,
rte_ring_sc_dequeue_bulk is replaced with rte_ring_sc_dequeue_burst.

Fixes: de3cfa2c9823 ("sched: initial import")

Suggested-by: Tao Y Yang <tao.y.yang@intel.com>
Signed-off-by: Jasvinder Singh <jasvinder.singh@intel.com>