ALSA: spi: Constify snd_kcontrol_new items

Most of snd_kcontrol_new definitions are read-only and passed as-is.
Let's declare them as const for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-41-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/spi/at73c213.c

@@ -657,7 +657,7 @@ static int snd_at73c213_aux_capture_volume_info(
 			| (mask << 24) | (invert << 22))		\
 }
 
-static struct snd_kcontrol_new snd_at73c213_controls[] = {
+static const struct snd_kcontrol_new snd_at73c213_controls[] = {
 AT73C213_STEREO("Master Playback Volume", 0, DAC_LMPG, DAC_RMPG, 0, 0, 0x1f, 1),
 AT73C213_STEREO("Master Playback Switch", 0, DAC_LMPG, DAC_RMPG, 5, 5, 1, 1),
 AT73C213_STEREO("PCM Playback Volume", 0, DAC_LLOG, DAC_RLOG, 0, 0, 0x1f, 1),