ALSA: scarlett2: Disable autogain during phantom power state change

When phantom power is enabled or disabled, the autogain control cannot
be enabled until the interface has signalled that the change is
complete and the input is unmuted. Update those controls to be
read-only during this time.

Signed-off-by: Geoffrey D. Bennett <g@b4.vu>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/f49f7bf9358e1f20713d95d407d8d6a436859877.1703612638.git.g@b4.vu