ALSA: pcm: Simplify proc file destruction

The proc files are recursively freed by calling with the root
snd_info_entry object, so we don't have to keep each object for
releasing one by one.  Move the release of the PCM stream proc root at
the beginning, so that we can remove the redundant code and resource.

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