iio: chemical: atlas-sensor: correct DO-SM channels

IIO_CONCENTRATION channel for the DO-SM shouldn't be indexed as
there isn't more than one, and also ATLAS_CONCENTRATION_CHANNEL
macro scan_index define steps on the IIO_TIMESTAMP channel.

Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>
Fixes: a751b8e4 (iio: chemical: atlas-sensor: add DO-SM module support)
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>