scsi: imm: mark expected switch fall-throughs

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

Notice that, in this particular case, I placed all the "Phase N - ..."
comments on the same line as its corresponding switch case. The same
way in which similar comments appear in drivers/scsi/ppa.c. This makes
it possible to place the "fall through" annotations at the bottom of
each 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>