drivers/video: Store device operations table

Fix a bug which the device operation structure is not stored in
video_initialize().

drivers/video/video.c

@@ -1499,6 +1499,8 @@ int video_initialize(FAR const char *devpath,
 
   video_handler = video_register(devpath);
 
+  g_video_devops = devops;
+
   is_initialized = true;
 
   return OK;
@@ -1513,6 +1515,8 @@ int video_uninitialize(void)
 
   video_unregister(video_handler);
 
+  g_video_devops = NULL;
+
   is_initialized = false;
 
   return OK;