ALSA: firewire-lib: unify packet handler for IT context

The handlers for packet with CIP and without CIP include common codes.
This commit unifies them and remove an member for pointer to callback
function from data structure.

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