scsi: qla2xxx: Introduce the dsd32 and dsd64 data structures

Introduce two structures for the (DMA address, length) combination instead
of using separate structure members for the DMA address and length. This
patch fixes several Coverity complaints about 'cur_dsd' being used to write
outside the bounds of structure members.

Cc: Himanshu Madhani <hmadhani@marvell.com>
Cc: Giridhar Malavali <gmalavali@marvell.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Acked-by: Himanshu Madhani <hmadhani@marvell.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>