media: ccs-pll: Make variables const where appropriate

Many functions operate on data structures that are read only, both in
the implementation and by design. Mark them as const to increase
readability and avoid future mistakes.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>