We leave the pcm field of struct hda_pcm at removal of each device, so
far.  This hasn't been a problem since unbinding the codec driver
isn't supposed to happen and another route via snd_hda_codec_reset()
clears all the once.  However, for a proper unbind implementation, we
need to care about it.

This patch does the thing above properly:

- Include struct hda_pcm pointer instead of struct hda_pcm_stream
  pointers in struct azx_dev.  This allows us to point the hda_pcm
  object at dev_free callback.

- Introduce to_hda_pcm_stream() macro for better readability.
Signed-off-by: Takashi Iwai <tiwai@suse.de>