thermal: tsens8974: fix setting negative thresholds

Fix setting negative temperature thresholds. The
errors are due to performing arithmetic math with
different type casts. Also fix the rounding errors
seen when setting temperature thresholds.

Change-Id: I3b98f603992a951e599224d6664c6cdad5a3d955
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>