ALSA: scarlett2: Add support for the internal "standalone" switch

The Focusrite Scarlett Gen 2/3 interfaces with internal mixers have a
"standalone" mode. When the interface is not connected to a USB host
and standalone mode is enabled, the interface will pass audio as
previously configured. This patch adds an ALSA control to allow
enabling/disabling that mode.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/cd88871c5e77abd5c23a4758a1f2ec9fd427fd69.1646578164.git.g@b4.vu


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