ALSA: usb-audio: scarlett2: Label 18i8 Gen 3 line outputs correctly

The 18i8 Gen 3 analogue 7/8 outputs are identified as line 3/4 on the
rear of the unit. Add support for remapping the channel numbers to
match the labelling.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/461acb911509e60e9ab48109ece3bbadae7440c8.1624379707.git.g@b4.vu


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