ACPI: thermal: Replace ternary operator with min_t()

Modify the code in accordance with the coccicheck warning:

drivers/acpi/thermal.c:422: WARNING opportunity for min().

min_t() macro is defined in include/linux/minmax.h. It avoids multiple
evaluations of the arguments when non-constant and performs strict
type-checking.

Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>