media: v4l: ioctl: Validate num_planes for debug messages

The num_planes field in struct v4l2_pix_format_mplane is used in a loop
before validating it. As the use is printing a debug message in this case,
just cap the value to the maximum allowed.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Cc: <stable@vger.kernel.org>      # for v4.12 and up
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>