dm: bugfix for iothread

As the type of pthread_t is unsigned long, so the init value for it
should not be '-1'. When the tid is '-1', it will continue to call
pthread_kill/pthread_join. This is incorrect.

Tracked-On: #7960
Signed-off-by: Conghui <conghui.chen@intel.com>
Reviewed-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Wang, Yu1 <yu1.wang@intel.com>

devicemodel/core/iothread.c

@@ -145,7 +145,7 @@ iothread_init(void)
 	pthread_mutex_init(&ioctx.mtx, &attr);
 	pthread_mutexattr_destroy(&attr);
 
-	ioctx.tid = -1;
+	ioctx.tid = 0;
 	ioctx.started = false;
 	ioctx.epfd = epoll_create1(0);