[SCSI] zfcp: Fix error checking for ELS ADISC requests

Correctly check the status for ELS ADISC requests. 0 means success.

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>