powerpc/pseries: Failed reconfig notifier chain call cleanup

The return code from invoking the notifier chain when updating the
ibm,dynamic-memory property is not handled properly. In failure
cases (rc == NOTIFY_BAD) we should be restoring the original value
of the property.  In success (rc == NOTIFY_OK) we should be returning
zero from the calling routine.

Signed-off-by: Nathan Fontenot <nfont@austin.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>