s390/sclp: handle default case in sclp memory notifier

When new memory notifier types are added in common code and
unimplemented on s390, return success instead of EINVAL return code.

This prevents breakage when new memory notifier types are added.

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Sumanth Korikkar <sumanthk@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>