iio: dummy_evgen: fix possible memleak in evgen init

The memory allocated in the function iio_dummy_evgen_create is not
released if it fails to add the evgen device to device hierarchy. This
may result in a memory leak bug.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>