[SCSI] zfcp: fix the data buffer accessor patch

Fix the data buffer accessor patch.

For request without a data buffer nothing was written into
a SBALE.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>