ALSA: hda: Fix max PCM level to 0 dB for AD1981_HP

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>