media: rkvdec: Fix .buf_prepare

The driver should only set the payload on .buf_prepare if the
buffer is CAPTURE type. If an OUTPUT buffer has a zero bytesused
set by userspace then v4l2-core will set it to buffer length.

If we overwrite bytesused for OUTPUT buffers, too, then
vb2_get_plane_payload() will return incorrect value which might be then
written to hw registers by the driver in rkvdec-h264.c.

[Changed the comment and used V4L2_TYPE_IS_CAPTURE macro]

Fixes: cd33c830 ("media: rkvdec: Add the rkvdec driver")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Andrzej Pietrasiewicz <andrzej.p@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>