watchdog: riowd: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings (Building: sparc64):

drivers/watchdog/riowd.c: In function ‘riowd_ioctl’:
drivers/watchdog/riowd.c:136:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   riowd_writereg(p, riowd_timeout, WDTO_INDEX);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/watchdog/riowd.c:139:2: 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>