ALSA: pcm: Don't add internal PCMs to PCM device list

An internal PCM object shouldn't be added to the PCM device list, as
it's never accessed directly from the user-space, and it has no proc
or any similar accesses.  Currently, it's excluded in snd_pcm_get()
and snd_pcm_next(), but it's easier not to add such an object to the
list.

Actually, the whole snd_pcm_dev_register() can be skipped for an
internal PCM.  So this patch changes the code there, but also
addresses the uninitialized list_head access.

Acked-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>