iio: imu: kmx61: Use correct base when reading data

We have two IIO devices and we need to adjust the base
when reading data.

Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Reviewed-by: Hartmut Knaack <knaack.h@gmx.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>