Added IEC958 (SPDIF) output support to Thinkpad T/X/Z60 with
AD1981HD codec.  The spdif jack is on docking station.
Also, renamed 'IEC958 Playback Route' to 'IEC958 Playback Source'
to avoid the mixer name confliction with IEC958 switch.
Signed-off-by: Takashi Iwai <tiwai@suse.de>