ALSA: hda - Don't access invalid substream in proc file

The commit e3303235
"ALSA: hda - proc - show which I/O NID is associated to PCM device"
introduces the access to substream pointer.  But, PCMs may have no
substreams in one or both directions, and this results in NULL
dereference.  Also, print the first substream number doesn't make
sense.

This patch removes the access to the substream pointer, and reformat
to fit to the standard coding style.

Signed-off-by: Takashi Iwai <tiwai@suse.de>