ALSA: firewire-lib: code refactoring for generation of syt sequence

This commit dissolves sequence generator in terms of syt offsets for
packet.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210522013303.49596-5-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>