bmi160: Make changes to align driver to datasheet

The logic in the driver was not aligned to the datasheet. Also,
temperature reading was not being done in fetch, but in channel_get.
There was also some extra conversions from SI->register->SI when
setting the range, this was causing the register value calculation to
produce an incorrect scale in some cases.

Tests were added to cover these cases.

Signed-off-by: Yuval Peress <peress@google.com>