[media] videobuf2: don't return POLLERR when only polling for events

If you only poll for events, then vb2_poll will return POLLPRI | POLLERR if
no streaming is in progress. That's not right, it's perfectly valid to
poll just for events.

Cc: Pawel Osciak <[e-mail]>
Cc: Marek Szyprowski <[e-mail]>
Cc: Kyungmin Park <[e-mail]>
Signed-off-by: Hans Verkuil <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

drivers/media/v4l2-core/videobuf2-core.c

···
 			poll_wait(file, &fh->wait, wait);
 	}
 
+	if (!V4L2_TYPE_IS_OUTPUT(q->type) && !(req_events & (POLLIN | POLLRDNORM)))
+		return res;
+	if (V4L2_TYPE_IS_OUTPUT(q->type) && !(req_events & (POLLOUT | POLLWRNORM)))
+		return res;
+
 	/*
 	 * Start file I/O emulator only if streaming API has not been used yet.
 	 */