staging: media: sunxi: cedrus: make vb2_ops struct definition const

This cleans up a checkstyle warning where the vb2_ops struct definitions
should be const.

Signed-off-by: Ian Cowan <ian@linux.cowan.aero>
Reviewed-by: Samuel Holland <samuel@sholland.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>