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

[ Upstream commit 7323d59862802ca109451eeda9777024a7625509 ]

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>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>