The way to maintain isochronous resources on bus is different between
Fireface 400/800.

This commit is a preparation. This commit moves a function to allocate resource to
model-dependent implementation.
Signed-off-by: NTakashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>