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 <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Tested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/874juzg3kd.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>