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>