ALSA: firewire-digi00x: refactoring to move timing of registration for isochronous channel

This commit is a part of preparation to perform allocation/release
of isochronous resources in pcm.hw_params/hw_free callbacks.

The registration of isochronous channels is done just after allocation
of isochronous resources. This commit separates the registration just
before starting packet streaming.

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