ALSA: usb-audio: Fix control 'access overflow' errors from chmap

The current channel-map control implementation in USB-audio driver may
lead to an error message like
  "control 3:0:0:Playback Channel Map:0: access overflow"
when CONFIG_SND_CTL_VALIDATION is set.  It's because the chmap get
callback clears the whole array no matter which count is set, and
rather the false-positive detection.

This patch fixes the problem by clearing only the needed array range
at usb_chmap_ctl_get().

Cc: <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20201211130048.6358-1-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/stream.c

@@ -193,16 +193,16 @@ static int usb_chmap_ctl_get(struct snd_kcontrol *kcontrol,
 	struct snd_pcm_chmap *info = snd_kcontrol_chip(kcontrol);
 	struct snd_usb_substream *subs = info->private_data;
 	struct snd_pcm_chmap_elem *chmap = NULL;
-	int i;
+	int i = 0;
 
-	memset(ucontrol->value.integer.value, 0,
-	       sizeof(ucontrol->value.integer.value));
 	if (subs->cur_audiofmt)
 		chmap = subs->cur_audiofmt->chmap;
 	if (chmap) {
 		for (i = 0; i < chmap->channels; i++)
 			ucontrol->value.integer.value[i] = chmap->map[i];
 	}
+	for (; i < subs->channels_max; i++)
+		ucontrol->value.integer.value[i] = 0;
 	return 0;
 }