scsi: aha1542: Clarify 'struct ccb' comments

This driver's original authors did pretty bad job of documenting the
Command Control Block (CCB) structure -- especially its 2nd byte, where the
bit numbers were completely left out. Sync up the 'struct ccb' comments to
the Adaptec AHA-154xA manual.

Link: https://lore.kernel.org/r/17a7be14-a9d2-9822-bb3e-1d7385f486b0@omprussia.ru
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Sergey Shtylyov <s.shtylyov@omprussia.ru>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>