pthread_mutexattr_getprotocol: modify prototype

modify the return value according to posix standard

Change-Id: I6e32a8f7a5cac85fe7a395a8115710fdff61d985
Signed-off-by: dongjiuzhu <dongjiuzhu1@xiaomi.com>

libs/libc/pthread/pthread_mutexattr_getprotocol.c

@@ -70,9 +70,11 @@ int pthread_mutexattr_getprotocol(FAR const pthread_mutexattr_t *attr,
 
 #ifdef CONFIG_PRIORITY_INHERITANCE
   linfo("Returning %d\n", attr->proto);
-  return attr->proto;
+  *protocol = attr->proto;
 #else
   linfo("Returning %d\n", PTHREAD_PRIO_NONE);
-  return PTHREAD_PRIO_NONE;
+  *protocol = PTHREAD_PRIO_NONE;
 #endif
+
+  return 0;
 }