ALSA: scarlett2: Allow for interfaces without per-channel volume

Currently-supported interfaces with a mixer have per-channel volume
controls, but this changes in Gen 4. Add a check so that the Playback
Volume and associated controls don't get created unless the
SCARLETT2_CONFIG_LINE_OUT_VOLUME config item is present.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/30f68cb311e27f2cc1351cb846218f7248a90263.1703444932.git.g@b4.vu


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