ALSA: usb-audio: scarlett2: Add support for the talkback feature

Add support for the talkback feature of the 18i20 Gen 3.

Co-developed-by: Vladimir Sadovnikov <sadko4u@gmail.com>
Signed-off-by: Vladimir Sadovnikov <sadko4u@gmail.com>
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/e39599893a7479c290e1aaec6c79dcee87681b47.1624379707.git.g@b4.vu
Signed-off-by: Takashi Iwai <tiwai@suse.de>