Thermal/dummy: Fix warning

```
CID 1159822: (#1 of 2): 宏将无符号值与 0 做了比较 (NO_EFFECT)
unsigned_compare: 不带符号值的此 greater-than-or-equal-to-zero 比较总是为 true。index >= 0U。
```
Signed-off-by: wangjianyu3 <wangjianyu3@xiaomi.com>

Author: JianyuWang0623

drivers/thermal/thermal_dummy.c

@@ -281,7 +281,7 @@ static int dummy_cpufreq_target_index(FAR struct cpufreq_policy *driver,
   FAR struct dummy_cpufreq_driver_s **dummy =
                                 (FAR struct dummy_cpufreq_driver_s **)driver;
 
-  DEBUGASSERT(index >= 0 && index < (*dummy)->table_len);
+  DEBUGASSERT(index < (*dummy)->table_len);
 
   (*dummy)->current = (*dummy)->table[index].frequency;
   return 0;