ALSA: usb-audio: scarlett2: Add Gen 3 MSD mode switch

Add a control to disable the Gen 3 MSD mode so that the full
functionality of the device is available. Don't create the other
controls until MSD mode is disabled.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/1cb93bbe585f6b0a74f5dc27450bc87e1f3776dc.1624379707.git.g@b4.vu


Signed-off-by: Takashi Iwai <tiwai@suse.de>