staging:iio:hmc5843: register <-> value arrays now can have different lengths

Changed structure of struct hmc5843_chip_info to include length of translation
arrays. Code previously using #defined constant has been changed accordingly.
This allows to integrate devices which do have different amounts of available
rates/scales.

Signed-off-by: Josef Gajdusek <atx@atx.name>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>