media: v4l2-compat-ioctl32: memset stack union in compat ioctl

memset karg in do_video_ioctl to ensure that stack memory is
not copied to user memory.

Change-Id: Ib892f8cabff1e0076c670496ee6353d00afdf85e
Signed-off-by: Surajit Podder <spodder@codeaurora.org>