ALSA: scarlett2: Add config set struct

Add struct scarlett2_config_set so that data which is common to all
devices in a config set can be stored there rather than in the
model-specific data.

Accordingly, rename scarlett2_config_items[] to
scarlett2_config_sets[].

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/bfdb04cd6239af9a8c26a52da0537980f77c0437.1703444932.git.g@b4.vu


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