ALSA: firewire-lib: fix data block counter for incoming packet without CIP header

The value of data block counter is not calculated for incoming packet
without CIP header. This commit fixes the bug.

Fixes: 947b437e

 ("ALSA: firewire-lib: unify packet handler for IR context")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>