ALSA: usb-audio: scarlett2: Add direct monitor support

The Solo and 2i2 devices don't have a mixer but they do have a "direct
monitor" switch. Add support for getting and setting the state of this
switch.

Co-developed-by: Vladimir Sadovnikov <sadko4u@gmail.com>
Signed-off-by: Vladimir Sadovnikov <sadko4u@gmail.com>
Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/61d23dc4feb3b046d870ad7203e66ff2bd1d278c.1624379707.git.g@b4.vu


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