sched/getprioritymax: handle invaild policy as posix style

modify the return value according to posix standard
------------
SCHED_GET_PRIORITY_MAX(2)

NAME
       sched_get_priority_max, sched_get_priority_min  - get static priority range

RETURN VALUE
       On success, sched_get_priority_max() and sched_get_priority_min() return the
                   maximum/minimum priority value for the named scheduling policy.
       On error, -1 is returned, and errno is set appropriately.

ERRORS
       EINVAL The argument policy does not identify a defined scheduling policy.

Signed-off-by: chao an <anchao@xiaomi.com>

libs/libc/sched/sched_getprioritymax.c

@@ -52,6 +52,11 @@
 
 int sched_get_priority_max(int policy)
 {
-  DEBUGASSERT(policy >= SCHED_FIFO && policy <= SCHED_OTHER);
+  if (policy < SCHED_FIFO || policy > SCHED_OTHER)
+    {
+      set_errno(EINVAL);
+      return ERROR;
+    }
+
   return SCHED_PRIORITY_MAX;
 }