thermal: Fix sensor thresholds not accounted correctly

Sensor threshold min and max are calculated to be binding around the
current temp, but they fail if there no thresholds available with the
min < curr_temp and the max > curr_temp.

Fix negative temperatures handling.

Change-Id: I124d2a9249f705d41469b8e0efffe2dfdf05e292
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>