thermal: tools: tmon: remove unneeded local variable

Fix the following coccicheck review:
/tools/thermal/tmon/pid.c:57:5-8: Unneeded variable

Remove unneeded variable used to store return value.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: ran jianping <ran.jianping@zte.com.cn>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>