Remove logically dead code and mark switch cases where we are expecting
to fall through.

Fix the following warnings (Building: defconfig sh):

arch/sh/kernel/disassemble.c:478:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
arch/sh/kernel/disassemble.c:487:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
arch/sh/kernel/disassemble.c:496:8: warning: this statement may fall
through [-Wimplicit-fallthrough=]
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>