ALSA: firewire-lib: fix check for the size of isochronous packet payload

ALSA: firewire-lib: fix check for the size of isochronous packet payload

The check for size of isochronous packet payload just cares of the size of
IR context payload without the size of CIP header.

Cc: <stable@vger.kernel.org>
Fixes: de8462205eda ("ALSA: firewire-lib: use 16 bytes IR context header to separate CIP header")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210513125652.110249-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>