bmi160: fix value calculation

As an example, when the gyro range is set to 250 deg/sec the scale
was set to 133 where it really should be 133.160058662. This leads
to a 0.12% error in the value returned. By separating the numerator
and denominator, we're able to drastically reduce the error.

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