ALSA: pcm: Fix the silence data for DSD formats

Right now we set 0 as the silence data for DSD_U8 and DSD_U16 formats,
but this is actually wrong.  0 is rather the most negative value.
Alternatively, we may take the repeating 0x69 pattern like ffmpeg
deploys.

Reference: https://ffmpeg.org/pipermail/ffmpeg-cvslog/2014-April/076427.html
Suggested-by: Alexander E. Patrakov <patrakov@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/core/pcm_misc.c

@@ -142,11 +142,11 @@ static struct pcm_format_data pcm_formats[(INT)SNDRV_PCM_FORMAT_LAST+1] = {
 	},
 	[SNDRV_PCM_FORMAT_DSD_U8] = {
 		.width = 8, .phys = 8, .le = 1, .signd = 0,
-		.silence = {},
+		.silence = { 0x69 },
 	},
 	[SNDRV_PCM_FORMAT_DSD_U16_LE] = {
 		.width = 16, .phys = 16, .le = 1, .signd = 0,
-		.silence = {},
+		.silence = { 0x69, 0x69 },
 	},
 	/* FIXME: the following three formats are not defined properly yet */
 	[SNDRV_PCM_FORMAT_MPEG] = {