thermal: msm_thermal: Fix for reading incorrect negative temperatures

tsens_get_temp() takes in an unsigned long, but the temperatures could
be negative as well. The sign could be intrepreted as large positive
value when using an unsigned long. Use long variable type instead.

CRs-Fixed: 441632
Change-Id: Iea0713c4809818af0e409747d4be4631eb4837e9
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>