ide: hpt366: 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 removed the "else: "
part from the comment. As we are now using Warning level 3:
-Wimplicit-fallthrough=3, this is what GCC is expecting to
find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>