[SCSI] zfcp: Remove sysfs attribute port_add

With the automatic scanning of remote ports in place, there is no need
to add remote ports manually. So, remove the port_add attribute.

Signed-off-by: Martin Peschke <mp3@de.ibm.com>
Signed-off-by: Christof Schmitt <christof.schmitt@de.ibm.com>
Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>