ALSA: dice: fix to wait for releases of all ALSA character devices

In a development period for Linux kernel v4.20, drivers in ALSA firewire
stack were changed to wait for releases of all ALSA character devices at
.remove callback of bus driver. However, ALSA dice driver is partly out
of this change. This bug can bring fault to user process which holds
the last of character device in unplugging.

This commit fixes the driver to wait in the callback.

Fixes: 61ccc6f6 ('ALSA: firewire: block .remove callback of bus driver till all of ALSA character devices are released')
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>