ALSA: usb-audio: scarlett2: Fix 6i6 Gen 2 line out descriptions

There are two headphone outputs, and they map to the four analogue
outputs.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/205e5e5348f08ded0cc4da5446f604d4b91db5bf.1624294591.git.g@b4.vu


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