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>