ALSA: scarlett2: Simplify enums by removing explicit values

This commit removes the explicit integer assignments from the enums.
The actual values matter little, and not assigning explicit values
makes it easier to modify the longer lists in the future.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/67f0f1bb8b90d7c76dfe7062d22d33bbde19cf93.1703444932.git.g@b4.vu


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