ALSA: firewire-digi00x: delayed registration of sound card

When some digi00x units are connected sequentially, userspace
applications are involved at bus-reset state on IEEE 1394 bus. In the
state, any communications can be canceled. Therefore, sound card
registration should be delayed till the bus gets calm.

This commit achieves it.

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