ALSA: firewire-motu: fix register handling for 896

After further investigation, I find out some mistakes for 896 about its
register. This commit fixes it.

Fixes: b431f16f

 ("ALSA: firewire-motu: add support for MOTU 896")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210623075941.72562-10-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>