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>

drivers/watchdog/wdt285.c

@@ -168,7 +168,7 @@ static long watchdog_ioctl(struct file *file, unsigned int cmd,
 		soft_margin = new_margin;
 		reload = soft_margin * (mem_fclk_21285 / 256);
 		watchdog_ping();
-		/* Fall */
+		/* Fall through */
 	case WDIOC_GETTIMEOUT:
 		ret = put_user(soft_margin, int_arg);
 		break;