staging: iio: hmc5843 change ABI to comply with documentation

this one completes my last patch set to compile fine for all drivers.

Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Datta, Shubhrajyoti <shubhrajyoti@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>