ALSA: pcm: avoid nused-but-set-variable warning

It will indicate below warning if W=1 was added and CONFIG_SND_DEBUG
was not set. This patch adds __maybe_unused and avoid it.

${LINUX}/sound/core/pcm_native.c: In function 'constrain_mask_params':
${LINUX}/sound/core/pcm_native.c:291:25: error: variable 'old_mask' set but not used [-Werror=unused-but-set-variable]
291 | struct snd_mask old_mask;
| ^~~~~~~~
${LINUX}/sound/core/pcm_native.c: In function 'constrain_interval_params':
${LINUX}/sound/core/pcm_native.c:327:29: error: variable 'old_interval' set but not used [-Werror=unused-but-set-variable]
327 | struct snd_interval old_interval;
| ^~~~~~~~~~~~
${LINUX}/sound/core/pcm_native.c: In function 'constrain_params_by_rules':
${LINUX}/sound/core/pcm_native.c:368:29: error: variable 'old_interval' set but not used [-Werror=unused-but-set-variable]
368 | struct snd_interval old_interval;
| ^~~~~~~~~~~~
${LINUX}/sound/core/pcm_native.c:367:25: error: variable 'old_mask' set but not used [-Werror=unused-but-set-variable]
367 | struct snd_mask old_mask;
| ^~~~~~~~
${LINUX}/sound/core/pcm_native.c: In function 'snd_pcm_hw_params_choose':
${LINUX}/sound/core/pcm_native.c:652:29: error: variable 'old_interval' set but not used [-Werror=unused-but-set-variable]
652 | struct snd_interval old_interval;
| ^~~~~~~~~~~~
${LINUX}/sound/core/pcm_native.c:651:25: error: variable 'old_mask' set but not used [-Werror=unused-but-set-variable]
651 | struct snd_mask old_mask;
| ^~~~~~~~
cc1: all warnings being treated as errors
make[3]: *** [${LINUX}/scripts/Makefile.build:250: sound/core/pcm_native.o] error 1

Signed-off-by: Kuninori Morimoto <[e-mail]>
Reviewed-by: Takashi Sakamoto <[e-mail]>
Tested-by: Takashi Sakamoto <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Takashi Iwai <[e-mail]>

sound/core/pcm_native.c

···
 					&substream->runtime->hw_constraints;
 	struct snd_mask *m;
 	unsigned int k;
-	struct snd_mask old_mask;
+	struct snd_mask old_mask __maybe_unused;
 	int changed;
 
 	for (k = SNDRV_PCM_HW_PARAM_FIRST_MASK; k <= SNDRV_PCM_HW_PARAM_LAST_MASK; k++) {
···
 					&substream->runtime->hw_constraints;
 	struct snd_interval *i;
 	unsigned int k;
-	struct snd_interval old_interval;
+	struct snd_interval old_interval __maybe_unused;
 	int changed;
 
 	for (k = SNDRV_PCM_HW_PARAM_FIRST_INTERVAL; k <= SNDRV_PCM_HW_PARAM_LAST_INTERVAL; k++) {
···
 	unsigned int stamp;
 	struct snd_pcm_hw_rule *r;
 	unsigned int d;
-	struct snd_mask old_mask;
-	struct snd_interval old_interval;
+	struct snd_mask old_mask __maybe_unused;
+	struct snd_interval old_interval __maybe_unused;
 	bool again;
 	int changed, err = 0;
 
···
 		-1
 	};
 	const int *v;
-	struct snd_mask old_mask;
-	struct snd_interval old_interval;
+	struct snd_mask old_mask __maybe_unused;
+	struct snd_interval old_interval __maybe_unused;
 	int changed;
 
 	for (v = vars; *v != -1; v++) {