ALSA: usb-audio: scarlett2: Add "air" switch support

Some inputs on Gen 3 models have an "air" feature which can be enabled
from the driver or (model-dependent) from the front panel. Add support
for getting and setting the state of those switches.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/44d448a4150b9c068754759c9fdd2bfe21484487.1624379707.git.g@b4.vu


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