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: 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>