[ALSA] hda-codec - Fix SPDIF output

Fix SPDIF output (at least on Realtek codecs).  The DIGI_CONVERT verbs
have to be reset before the PCM stream is set up.  Otherwise the digital
setup is screwed up.
Also, check the AMP capability before setting AMP of the digital out
widget.

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