watchdog: wdt285: Mark expected switch fall-through

Mark switch cases where we are expecting to fall through.

Fix the following warning (Building: footbridge_defconfig arm):

drivers/watchdog/wdt285.c: In function ‘watchdog_ioctl’:
drivers/watchdog/wdt285.c:170:3: warning: this statement may fall through [-Wimplicit-fallthrough=]
   watchdog_ping();
   ^~~~~~~~~~~~~~~
drivers/watchdog/wdt285.c:172:2: note: here
  case WDIOC_GETTIMEOUT:
  ^~~~

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>