ALSA: scarlett2: Disable input controls while autogain is running

While the autogain function is running, the other input controls
(select, link, gain, safe, level, air, and phantom) can't be modified.
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/ad13bacae77860de8c2d7c89f6ec2a1ee104e65f.1703612638.git.g@b4.vu