The dca_ops structure is never modified, so declare it as const.

Done with the help of Coccinelle.
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Acked-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NVinod Koul <vinod.koul@intel.com>