ALSA: hda: slave digital out support

Added support for playing a stream on multiple digital outs. This is done
by defining codec->slave_dig_outs as array of hda_nid_t with a null-terminated entry to set the
slave SPDIF outs, in which the slave outs have cloned settings of the master out (e.g. dig_out_nid).

Signed-off-by: Matthew Ranostay <mranostay@embeddedalley.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>

sound/pci/hda/hda_codec.h

@@ -725,6 +725,7 @@ struct hda_codec {
 	unsigned int spdif_status;	/* IEC958 status bits */
 	unsigned short spdif_ctls;	/* SPDIF control bits */
 	unsigned int spdif_in_enable;	/* SPDIF input enable? */
+	hda_nid_t *slave_dig_outs; /* optional digital out slave widgets */
 
 	struct snd_hwdep *hwdep;	/* assigned hwdep device */