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

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>
Cc: <stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>