net_sched: Add flow control support to prio qdisc

Add enable_flow flag to the prio qdisc.  Packet flow is
enabled by default, but can be disabled from userspace
(e.g. IPROUTE2 tc tool).  This allows for suspending packet
dequeue on a per-qdisc basis, which is needed to supprot
Quality of Service (QOS) when using WWAN modem.

Change-Id: I932f296be946f1acc3b00c7d8569bbb733d33622
Acked-by: Andrew Richardson <randrew@qualcomm.com>
CRs-Fixed: 283471
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>