ALSA: HDA: Add a new Conexant codec ID (506c)

Conexant ID 506c was found on Acer Aspire 3830TG. As users report
no playback, sending to stable should be safe.

Cc: stable@kernel.org
BugLink: https://bugs.launchpad.net/bugs/783582


Reported-by: andROOM
Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>