iio: industrialio-buffer: Fix iio_buffer_poll return value

commit 1bdc0293 upstream.

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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

drivers/iio/industrialio-buffer.c

@@ -67,7 +67,7 @@ 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 (rb->stufftoread)