scsi: remove current_cmnd field from struct scsi_device

The field is only used by the 53c700 driver, so move it into the
driver-private device data instead of having it in the common structure.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tomas Henzl <thenzl@redhat.com>
Reviewed-by: Ewan D. Milne <emilne@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>