scsi_debug: scsi_cmnd->cmnd check and casts unnecessary

This patch removes a NULL check for the scsi_cmnd::cmnd pointer
since many other instances in this driver and elsewhere assume
it is valid. Also redundant casts to 'unsigned char *' are removed
as the pointer has that type.

Signed-off-by: Douglas Gilbert <dgilbert@interlog.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>