[SCSI] zfcp: verify for correct rport state before scanning for SCSI devs

Prevent a SCSI target scan for a rport which have turned invalid
in the meantime.

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