ENGR00266881 mxc vout:Flush workqueue when change is needed for streaming

ENGR00266881 mxc vout:Flush workqueue when change is needed for streaming

We currently call cancel_work_sync() to have all left work be done. But,
this is not safe to make sure all left work being done successfully.
Instead, chances are that some work may be cancelled before starting
to be done, which may cause frame lost and make us hang at upcoming
wait_event_interruptible() in videobuf_waiton() called from video buffer
core v1 framework's dqbuf API. This patch replaces the function
call cancel_work_sync() with flush_workqueue() to fix the issue.

Signed-off-by: Liu Ying <Ying.Liu@freescale.com>