staging: iio: Function iio_get_new_idr_val() return negative value if fails.

Function iio_get_new_idr_val() return negative value if fails.
So, only error when ret < 0 in iio_device_register_eventset().

Signed-off-by: Sonic Zhang <sonic.adi@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>