ALSA: hda - Fix / clean-up slave digital out codes

The recent slave_dig_out addition has some rooms to clean up.
Also it doesn't call snd_hda_cleanup_stream() properly for slaves
at closing.  The patch fixes both issues.

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