s390/defconfig: set SCSI_DH=y

Fix this warning:
arch/s390/configs/performance_defconfig:380:warning: symbol value 'm' invalid for SCSI_DH

Introduced via 086b91d0


(scsi_dh: integrate into the core SCSI code)

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