iio: adis16400: Use != channel indices for the two voltage channels

commit 7323d59862802ca109451eeda9777024a7625509 upstream.

Previously, the two voltage channels had the same ID, which didn't cause
conflicts in sysfs only because one channel is named and the other isn't;
this is still violating the spec though, two indexed channels should never
have the same index.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>