staging:iio:hmc5843: Use SCALE instead of magn_range

v3:
* rename _check_scale() to _get_scale_index()
v2:
* use SCALE instead of CALIBSCALE to control the range/gain
  of measurements

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>