msm: thermal: Use signed int for temperature

Temperatures could be in negative and negative signed ints may be
incorrectly read as unsigned int.

Change-Id: I817db2756c54816296a700ffcd004a83161f2b95
CRs-Fixed: 441632, 473507
Signed-off-by: Praveen Chidambaram <pchidamb@codeaurora.org>