s390/ccwgroup: fix an uninitialized return code

Since commit 0b60f9ea
"s390: use device_remove_file_self() instead of device_schedule_callback()"
the return code of ccwgroup_ungroup_store is uninitialized. Make
sure the rc is always initialized.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>