ALSA: scarlett2: Add scarlett2_mixer_value_to_db()

Refactor scarlett2_usb_get_mix(), moving the scarlett2_mixer_values[]
lookup into scarlett2_mixer_value_to_db().

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/7adf869852aba2819fddb850b0ea8df5f7d73931.1703444932.git.g@b4.vu


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