[SCSI] zfcp: Dont clear reference from SCSI device to unit

It is possible that a remote port has a problem, the SCSI device gets
deleted after the rport timeout and then the timeout for pending SCSI
commands trigger an abort. For this case, don't delete the reference
from the SCSI device to the zfcp unit, so that we can still have the
reference to issue an abort request.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Swen Schillig <swen@vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>