[SCSI] zfcp: remove DID_DID flag

The port flag DID_DID indicates whether we know the current id of the
port. This is always set in parallel. Since the id 0 is invalid
(because the port id 0 is invalid) we can remove the DID_DID flag:
d_id of 0 indicates an invalid d_id != 0 is a valid one.

Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Acked-by: Felix Beck <felix@linux.vnet.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>