thermal: thermal-core: Change temp type from unsigned to int

Negative temperature values can be set by thermal clients
to the TSENS driver. Having unsgined long type prevents
clients using the set_trip_temp() thermal api to set
negative temperature values. On msm-3.4 kernel this api
had support to set negative temperature. Change the type
to support the negative temperature.

Change-Id: I987aed5a291b76d8360a9256cad0ff0c1adb06c8
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>