ALSA: scarlett2: Remove scarlett2_config_sets array

Replace array index into config sets with a pointer to a config set.
Copy the config_set pointer to the scarlett2_data struct.

This simplifies both the definition and use of the config sets.
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/61f69519fb6fbb677e066891a3a6771aeeec106d.1703444932.git.g@b4.vuSigned-off-by: Takashi Iwai <tiwai@suse.de>