tools/power/acpi: close file only if it is open

The logic on the test for a valid fd to close is incorrect.
This was just a mistake and was pointed out by Colin Ian King.

Link: https://patchwork.kernel.org/patch/8620201/


Original-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>