ACPI: thermal_lib: Initialize temp_decik to zero

Static analysis with clang scan build is warning that uninitialized
data is being passed into various functions. Stop these warnings by
initializing temp_decik to zero.

Cleans up clang scan warnings in lines 106, 125, 146 and 164 such as:
drivers/acpi/thermal_lib.c:106:9: warning: 2nd function call argument
is an uninitialized value [core.CallAndMessage]

Kudos to Dan Carpenter for the deeper analysis of this issue.

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>