scsi: NCR5380: Avoid compiler warning when -Wimplicit-fallthrough is enabled

Adjust comments accordingly.

Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Michael Schmitz <schmitzmic@gmail.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>