Bluetooth: mark expected switch fall-throughs

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

In this particular case, notice that I replaced the
"deliberate fall-through..." comment with a "fall through"
comment, which is what GCC is expecting to find.

Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>