[SCSI] zfcp: Correctly query end flag in gpn_ft response

When the last port entry in the gpn_ft response is already a known
port, the eval function would skip the end flag and try to attach
ports with wwpn 0. Fix this by checking the flag first.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: Martin Petermann <martin@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>