ALSA: scarlett2: Store mix_ctls for Gen 4 Direct Monitor

The Scarlett 4th Gen small interfaces have a software-controllable
mixer like the large 2nd and 3rd Gen interfaces do. Pressing the
"Direct" button on the interface updates the mixer controls, which
this driver hasn't needed to deal with previously.

This commit stores the ALSA mixer controls, and adds a mix_updated
flag so that the controls can be updated when a 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/3ba27c60230511b80b0fa75727551ea70f17d829.1703612638.git.g@b4.vu