ALSA: hda - Enable docking-station SPDIF for Thinkpad

The docking-station of Thinkpad X200 & co supports also an SPDIF output,
and the corresponding pin 0x1c has to be enabled for using it.

Reported-and-tested-by: Sebastian Glita <sebastian.glita@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>