scsi: wd33c93: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: m68k):

drivers/scsi/wd33c93.c: In function  round_4 :
drivers/scsi/wd33c93.c:1856:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
   case 2: ++x;
           ^~~
drivers/scsi/wd33c93.c:1857:3: note: here
   case 3: ++x;
   ^~~~

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