cciss: clarify command list padding calculation

#define CMD_MSG_STALE	0xff

/* This structure needs to be divisible by 8 for new

 * indexing method.

 * indexing method.  PAD_32 and PAD_64 can be adjusted

 * independently as needed for 32-bit and 64-bits systems.

 */

#define PADSIZE (sizeof(long) - 4)

#define IS_64_BIT ((sizeof(long) - 4)/4)

#define IS_32_BIT (!IS_64_BIT)

#define PAD_32 (0)

#define PAD_64 (4)

#define PADSIZE (IS_32_BIT * PAD_32 + IS_64_BIT * PAD_64)

typedef struct _CommandList_struct {

  CommandListHeader_struct Header;

  RequestBlock_struct      Request;