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>

arch/sh/kernel/kgdb.c

@@ -266,6 +266,7 @@ int kgdb_arch_handle_exception(int e_vector, int signo, int err_code,
 		ptr = &remcomInBuffer[1];
 		if (kgdb_hex2long(&ptr, &addr))
 			linux_regs->pc = addr;
+		/* fallthrough */
 	case 'D':
 	case 'k':
 		atomic_set(&kgdb_cpu_doing_single_step, -1);