media: mtk-vcodec: 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").

[hverkuil: drop unused variable i]

Cc: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>