ALSA: usb-audio: scarlett2: Add sw_hw_ctls and mux_ctls

Save the struct snd_kcontrol pointers for the sw_hw and mux controls.
This is in preparation for speaker switching support which needs to be
able to update those controls.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/269d89181bf29dbea80ba6f8cfff84fb23b77f86.1624379707.git.g@b4.vu


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