kdb: kdb_keyboard: mark expected switch fall-throughs

kdb: kdb_keyboard: 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 replaced the code comments with
a proper "fall through" annotation, which is what GCC is expecting
to find.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>