ALSA: scarlett2: Split dim_mute_update from vol_updated

Scarlett Gen 2 and Gen 3 devices combine volume and dim/mute
notifications. The Scarlett 4i4 Gen 4 has volume change notification
but no dim/mute control so split dim_mute_update out from vol_update.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/bf63f48bcc68ae739bd9948c8ee2f87ee7af22a2.1703444932.git.g@b4.vu


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