ALSA: gus: 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: Takashi Iwai <tiwai@suse.de>

sound/isa/gus/gus_pcm.c

@@ -562,14 +562,14 @@ static snd_pcm_uframes_t snd_gf1_pcm_playback_pointer(struct snd_pcm_substream *
 	return pos;
 }
 
-static struct snd_ratnum clock = {
+static const struct snd_ratnum clock = {
 	.num = 9878400/16,
 	.den_min = 2,
 	.den_max = 257,
 	.den_step = 1,
 };
 
-static struct snd_pcm_hw_constraint_ratnums hw_constraints_clocks  = {
+static const struct snd_pcm_hw_constraint_ratnums hw_constraints_clocks  = {
 	.nrats = 1,
 	.rats = &clock,
 };