When introducing the list of packet descriptor, for rx packet, the
calculation of scheduled isoc cycle is omitted. This commit fixes the
bug.

Fixes: f4f6ae7b ("ALSA: firewire-lib: use packet descriptor for IT context")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>