dm: iothread: fix bug in iothread handler

Fix the bug in iothread handler, the event should be read out so that the
next epoll_wait not return directly as the fd can still readable.

Tracked-On: #8181
Signed-off-by: Conghui <conghui.chen@intel.com>
Acked-by: Wang, Yu1 <yu1.wang@intel.com>

devicemodel/include/iothread.h

@@ -10,6 +10,7 @@
 struct iothread_mevent {
 	void (*run)(void *);
 	void *arg;
+	int fd;
 };
 int iothread_add(int fd, struct iothread_mevent *aevt);
 int iothread_del(int fd);