ALSA: firewire-lib: fix calculation for size of IR context payload

The quadlets for CIP header is handled as a part of IR context header,
thus it doesn't join in IR context payload. However current calculation
includes the quadlets in IR context payload.

Cc: <stable@vger.kernel.org>
Fixes: f11453c7

 ("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/20210513125652.110249-5-o-takashi@sakamocchi.jp


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