net: sched: Schedule PRIO qdisc when flow control released

The PRIO qdisc supports flow control, such that packet
dequeue can be disabled based on boolean flag 'enable_flow'.
When flow is re-enabled, the latency for new packets
arriving at network driver is high.  To reduce the delay in
scheduling packets, the qdisc will now invoke
__netif_schedule() to expedite dequeue.  This significantly
reduces the latency of packets arriving at network driver.

Change-Id: Ic5fe3faf86f177300d3018b9f60974ba3811641c
CRs-Fixed: 355156
Acked-by: Jimi Shah <jimis@qualcomm.com>
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>