media: staging/intel-ipu3: Do not zero reserved fields

Core code already clears reserved fields of struct v4l2_pix_format_mplane,
check commit 4e1e0eb0 ("media: v4l2-ioctl: Zero v4l2_plane_pix_format
reserved fields").

Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>