scsi: NCR5380: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
fall-through warning by replacing a /* fallthrough */ comment with the new
pseudo-keyword macro fallthrough;

Link: https://github.com/KSPP/linux/issues/115
Link: https://lore.kernel.org/r/20210604022752.GA168289@embeddedor


Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>