ALSA: firewire-motu: code refactoring for MOTU data block processing layer

This is code refactoring for MOTU data block processing layer so that
it can receive list of packet descriptor.

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