ALSA: firewire-motu: fix register handling for 828

After further investigation, I find out some mistakes for 828 about its
register. This commit fixes it.

Fixes: d13d6b28

 ("ALSA: firewire-motu: add support for MOTU 828")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210623075941.72562-9-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>