iio: industrialio-buffer: Fix iio_buffer_poll return value

Change return value to 0 if no device is bound since
unsigned int cannot support negative error codes.

Fixes: f18e7a06 ("iio: Return -ENODEV for file operations if the
device has been unregistered")
Signed-off-by: Cristina Opriceana <cristina.opriceana@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

drivers/iio/industrialio-buffer.c

@@ -164,7 +164,7 @@ unsigned int iio_buffer_poll(struct file *filp,
 	struct iio_buffer *rb = indio_dev->buffer;
 
 	if (!indio_dev->info)
-		return -ENODEV;
+		return 0;
 
 	poll_wait(filp, &rb->pollq, wait);
 	if (iio_buffer_ready(indio_dev, rb, rb->watermark, 0))