commit 41d903c0 upstream.

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>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>