usb: gadget: midi2: Add "Operation Mode" control

Add a new ALSA control element to watch the current operation mode
(MIDI 1.0 or MIDI 2.0).  It's a read-only control that reflects the
current value of altsetting, and 0 means unused, 1 for MIDI 1.0
(altset 0) and 2 for MIDI 2.0 (altset 1).

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20230725062206.9674-7-tiwai@suse.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>