ALSA: usb-audio: scarlett2: Remove incorrect S/PDIF comment

The 18i8 Gen 2 S/PDIF outputs are available at 192kHz, unlike
the 18i20 Gen 2. Remove the comment that says otherwise.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/20210620164622.GA9155@m.b4.vu


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