BugLink: https://bugs.launchpad.net/bugs/461062

The original reporter states that PCM maxes at +12 dB and results in
very bad distortion.  Cap PCM at 0 dB to resolve this symptom.
Signed-off-by: Daniel T Chen <crimsun@ubuntu.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>