ALSA: firewire-lib: fix isoc cycle count to which rx packet is scheduled

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>