ALSA: firewire-motu: parse messages for mixer source parameters in register-DSP model

ALSA: firewire-motu: parse messages for mixer source parameters in register-DSP model

In register DSP models, current parameters of DSP are always reported by
messages in isochronous packet. When user operates hardware component such
as rotary knob, corresponding message is changed.

This commit parses the message and cache current parameters of mixer
source function, commonly available for all of register DSP models.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211015080826.34847-5-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>