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@embeddedorReviewed-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>

drivers/scsi/NCR5380.c

@@ -1816,7 +1816,7 @@ static void NCR5380_information_transfer(struct Scsi_Host *instance)
 				switch (tmp) {
 				case ABORT:
 					set_host_byte(cmd, DID_ABORT);
-					/* fallthrough */
+					fallthrough;
 				case COMMAND_COMPLETE:
 					/* Accept message by clearing ACK */
 					sink = 1;