ALSA: usb-audio: Input source control - digidesign mbox

ALSA: usb-audio: Input source control - digidesign mbox

This adds a second mixer control to Digidesign Mbox
to select between Analog/SPDIF capture.

Users will note that selecting the SPDIF input source
automatically switches the clock mode to sync to SPDIF,
which is a feature of the hardware.

(You can change the clock source back to internal if you want
to capture from spdif but not sync to its clock although this mode
is probably not recommended).

Unfortunately, starting the stream resets both modes
to Internally clocked and Analog input mode.

Signed-off-by: Damien Zammit <damien@zamaudio.com>
Tested-by: Damien Zammit <damien@zamaudio.com>
Link: https://lore.kernel.org/r/20210813113402.11849-1-damien@zamaudio.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>