[SCSI] zfcp: Remove ZFCP_DID_MASK

Instead of assigning 4 bytes with the highest byte masked out, use a 3
byte array with the ntoh24 and h24ton helper functions, thus
eliminating the need for the ZFCP_DID_MASK.

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