sh: kgdb: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following error:

LINUX/arch/sh/kernel/kgdb.c: In function 'kgdb_arch_handle_exception':
LINUX/arch/sh/kernel/kgdb.c:267:6: error: this statement may fall through [-Werror=implicit-fallthrough=]
if (kgdb_hex2long(&ptr, &addr))
^
LINUX/arch/sh/kernel/kgdb.c:269:2: note: here
case 'D':
^~~~

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>