media: mediatek: vcodec: Constify struct vb2_ops

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 <christophe.jaillet@wanadoo.fr>
Signed-off-by: Sebastian Fricke <sebastian.fricke@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>