ALSA: firewire-motu: add ioctl command to read cached hardware meter

ALSA: firewire-motu: add ioctl command to read cached hardware meter

This patch adds new ioctl commands for userspace applications to read
cached image about hardware meters in register DSP and command DSP models.

The content of image differs depending on models. Model-specific parser
should be implemented in userspace.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211015080826.34847-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>