net: sched: export an api to enable/disable flow on sch

Export a function from sch_api.c that will look up
desired qdisc and call it's registered change function
to enable/disable flow.

Change-Id: I5b6dc7a6fd2b09b796c92b3770ba83423d19c864
CRs-Fixed: 355156
Acked-by: Jimi Shah <jimis@qualcomm.com>
Signed-off-by: Tianyi Gou <tgou@codeaurora.org>