ALSA: firewire-motu: queue event for parameter change in register DSP model

This commit is a preparation to notify parameter change of register DSP
to userspace application. A simple queue is added to store encoded data
for the change as long as ALSA hwdep character device is opened by
application.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211015080826.34847-11-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>