watchdog: wdt977: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

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

drivers/watchdog/wdt977.c: In function ‘wdt977_ioctl’:
  LD [M]  drivers/media/platform/vicodec/vicodec.o
drivers/watchdog/wdt977.c:400:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   wdt977_keepalive();
   ^~~~~~~~~~~~~~~~~~
drivers/watchdog/wdt977.c:403:2: 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>