ALSA: scarlett2: Handle Gen 4 Direct Monitor mix updates

When the Direct Monitor feature on the Scarlett 4th Gen Solo and 2i2
interfaces is used, the Mix A and B gains are updated by the
interface. This patch calls snd_ctl_notify() for the ALSA mix controls
when a Direct Monitor notification is received.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/713d032e343e0547212368919bef17d6fa1c9d29.1703612638.git.g@b4.vu