According to the reporter, node 0x15 needs to be muted for subwoofer
to stop sounding. This pin is marked as unused by BIOS, so fix that.

BugLink: http://bugs.launchpad.net/bugs/715877

Cc: stable@kernel.org (2.6.37+)
Reported-by: Hans Peter
Signed-off-by: NDavid Henningsson <david.henningsson@canonical.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>