scsi: aic7xxx: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that, in some cases, I replaced "FALLTHROUGH" with a "fall through"
annotation and then placed it at the bottom of the corresponding switch
case, which is what GCC is expecting to find.

Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>