media: cx231xx: 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
  36607	   1156	      0	  37763	   9383	drivers/media/usb/cx231xx/cx231xx-417.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  36735	   1016	      0	  37751	   9377	drivers/media/usb/cx231xx/cx231xx-417.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>