ALSA: dice: simplify unit probe processing

In former commit, ALSA dice driver doesn't generate kernel warnings
when unplugging units before initializing stream data.

This commit moves the initialization to delayed registration of sound
card, to simplify unit probe processing.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>