ALSA: scarlett2: Add support for custom Gen 4 Direct Monitor mixes

The mixes used by Direct Monitor feature on the Scarlett 4th Gen Solo
and 2i2 interfaces are configurable. This patch adds ALSA controls for
the gains which are copied to the mixer when Direct Monitor is
enabled.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/96282a805b45f04560e5923d170745363906b7f3.1703612638.git.g@b4.vu