media: pxa_camera: constify v4l2_clk_ops structure

This v4l2_clk_ops structure is only passed as the first argument of
v4l2_clk_register, which is const, so the v4l2_clk_ops structure can
also be const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>