Read only one of the GPIO pins as an analog voltage. The ADC can be
switched to a different GPIO pin at runtime, but this is not supported.

Previously, this driver would report the analog voltage of the currently
selected GPIO pin as all three GPIO voltages: in9_input, in10_input and
in11_input.
Signed-off-by: NIra W. Snyder <iws@ovro.caltech.edu>
Signed-off-by: NJean Delvare <khali@linux-fr.org>
Cc: stable@kernel.org