ALSA: fireface: allocate isochronous resources in mode-specific implementation

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