ALSA: usb-audio: scarlett2: Update mux controls to allow updates

Enabling/disabling speaker switching will update the mux
configuration. To prepare for this, add a private->mux_updated flag
and update the scarlett2_mux_src_enum_ctl_get() callback to check it.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Link: https://lore.kernel.org/r/5ce3bb9fe4006b550d18c783c5ff640fe0bfbfcb.1624379707.git.g@b4.vu


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