ALSA: es1688: Constify hw_constraints

snd_pcm_hw_constraint_list(), *_ratnums() and *_ratdens() receive the
const pointers.  Constify the corresponding static objects for better
hardening.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/isa/es1688/es1688_lib.c

@@ -290,7 +290,7 @@ static int snd_es1688_init(struct snd_es1688 * chip, int enable)
 
  */
 
-static struct snd_ratnum clocks[2] = {
+static const struct snd_ratnum clocks[2] = {
 	{
 		.num = 795444,
 		.den_min = 1,
@@ -305,7 +305,7 @@ static struct snd_ratnum clocks[2] = {
 	}
 };
 
-static struct snd_pcm_hw_constraint_ratnums hw_constraints_clocks  = {
+static const struct snd_pcm_hw_constraint_ratnums hw_constraints_clocks  = {
 	.nrats = 2,
 	.rats = clocks,
 };