scsi: aic94xx: Fix fall-through warning for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a
warning by explicitly adding a fallthrough; statement.

Notice that this seems to be a Duff device for performance[1]. So,
although the code looks a bit _funny_, I didn't want to refactor
or modify it beyond merely adding a fallthrough marking, which
might be the least disruptive way to fix this issue.

[1] https://www.drdobbs.com/a-reusable-duff-device/184406208

Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>

drivers/scsi/aic94xx/aic94xx_sds.c

@@ -718,11 +718,13 @@ static void *asd_find_ll_by_id(void * const start, const u8 id0, const u8 id1)
 	do {
 		switch (id1) {
 		default:
-			if (el->id1 == id1)
+			if (el->id1 == id1) {
+			fallthrough;
 		case 0xFF:
 				if (el->id0 == id0)
 					return el;
 			}
+		}
 		el = start + le16_to_cpu(el->next);
 	} while (el != start);