iio: event: Remove negative error code from iio_event_poll
Negative return values are not supported by iio_event_poll since
its return type is unsigned int.
Fixes: f18e7a068a0a3 ("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>