ALSA: pcm: Don't call register and disconnect callbacks for internal PCM

The internal PCM (aka DPCM backend PCM) doesn't need any registration
procedure, thus currently we bail out immediately at dev_register
callback.  Similarly, its counterpart, dev_disconnect callback, is
superfluous for the internal PCM.  For simplifying and avoiding the
conflicting disconnect call for internal PCM objects, this patch drops
dev_register and dev_disconnect callbacks for the internal ops.

The only uncertain thing by this action is whether skipping the PCM
state change to SNDRV_PCM_STATE_DISCONNECT for the internal PCM is
mandatory.  Looking through the current implementations, this doesn't
look so, hence dropping the whole dev_disconnect would make more
sense.

Tested-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>