media: mediatek: vcodec: Constify struct vb2_ops

"struct vb2_ops" are not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
text data bss dec hex filename
18059 3096 16 21171 52b3 drivers/media/platform/mediatek/vcodec/decoder/mtk_vcodec_dec_stateful.o

After:
=====
text data bss dec hex filename
18171 2968 16 21155 52a3 drivers/media/platform/mediatek/vcodec/decoder/mtk_vcodec_dec_stateful.o

Signed-off-by: Christophe JAILLET <[e-mail]>
Signed-off-by: Sebastian Fricke <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>

drivers/media/platform/mediatek/vcodec/decoder/mtk_vcodec_dec_stateful.c

···
 	}
 }
 
-static struct vb2_ops mtk_vdec_frame_vb2_ops = {
+static const struct vb2_ops mtk_vdec_frame_vb2_ops = {
 	.queue_setup = vb2ops_vdec_queue_setup,
 	.buf_prepare = vb2ops_vdec_buf_prepare,
 	.wait_prepare = vb2_ops_wait_prepare,

drivers/media/platform/mediatek/vcodec/decoder/mtk_vcodec_dec_stateless.c

···
 	return 0;
 }
 
-static struct vb2_ops mtk_vdec_request_vb2_ops = {
+static const struct vb2_ops mtk_vdec_request_vb2_ops = {
 	.queue_setup	= vb2ops_vdec_queue_setup,
 	.wait_prepare	= vb2_ops_wait_prepare,
 	.wait_finish	= vb2_ops_wait_finish,