scsi: ncr5380: Improve hostdata struct member alignment and cache-ability

Re-order struct members so that hot data lies at the beginning of the
struct and cold data at the end. Improve the comments while we're here.

Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Tested-by: Ondrej Zary <linux@rainbow-software.org>
Tested-by: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>