ALSA: hda - hdmi operate spdif based on pcm

Currently, the driver operates the spdif based on pin.
This is ok for the current driver as pcm is statically
bound to the pin.

However, if the driver uses dynamically pcm assignment,
this will cause confusion for user space.

The patch changes spdif operation from pin based to pcm based.

Signed-off-by: Libin Yang <libin.yang@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>