ALSA: usb-audio: scarlett2: Rename buttons/interrupts/vol

To match the vendor's terminology, change #defines, identifiers, and
comments:
- mute/dim/hardware buttons are now called dim/mute
- mixer status/interrupt is now notify
- vol is now monitor

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/20210620164636.GA9199@m.b4.vu


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