[ALSA] hda-codec - Fix AD1988 SPDIF output

The SPDIF output on AD1988 had some problems due to the wrongly routed
analog loopback to SPDIF.  This patch fixes the implementation of
'IEC958 Playback Source' mixer to handle the amp bits of mixer widget
0x1d correctly.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>