[S390] shutdown actions: save/return rc from init function

We always returned -EINVAL when setting of a shutdown action failed. This was
misleading, if for example the hardware did not support the shutdown action.
Now we save each shutdown action's init return code and return it when the
action is being set.

Signed-off-by: Frank Munzert <munzert@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>