[SCSI] zfcp: fix wrong handling of failed requests for GID_PN command

Fixes the handling of failed requests for GID_PN nameserver command:
Set ZFCP_STATUS_PORT_INVALID_WWPN only if indicated by response
payload for GID_PN nameserver command and not if fsf request fails.

Signed-off-by: Andreas Herrmann <aherrman@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>