Mark switch cases where we are expecting to fall through.

This patch fixes the following warning (Building: i386):

drivers/watchdog/scx200_wdt.c: In function ‘scx200_wdt_ioctl’:
drivers/watchdog/scx200_wdt.c:188:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   scx200_wdt_ping();
   ^~~~~~~~~~~~~~~~~
drivers/watchdog/scx200_wdt.c:189:2: note: here
  case WDIOC_GETTIMEOUT:
  ^~~~
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>