ALSA: usb-audio: scarlett2: Add Gen 3 mixer support

Add mixer support for the Focusrite Scarlett 4i4, 8i6, 18i8, and 18i20
Gen 3 devices.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/22d0dc877dec026eb19630edec217ab72ebcd50a.1624379707.git.g@b4.vu


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