ALSA: firewire-digi00x: code refactoring to keep isochronous resources

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

All of models in Digidesign Digi00x family have the same formation of
data channels in isochronous packet for both directions. This commit
simplifies allocation of isochronous resources in this point.

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