ALSA: usb-audio: scarlett2: Fix Level Meter control

The Level Meter control had a fixed number of channels and therefore
only worked with the 18i20 Gen 2. Fix the control to contain the
correct number of channels.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/5e9a2d5c136270db2d048db53a3b4b6e6d4a63de.1624294591.git.g@b4.vu
Signed-off-by: Takashi Iwai <tiwai@suse.de>