watchdog: Mark expected switch fall-throughs

Mark switch cases where we are expecting to fall through.

This patch fixes the following warnings:

drivers/watchdog/ar7_wdt.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 237:3
drivers/watchdog/pcwd.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 653:3
drivers/watchdog/sb_wdog.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 204:3
drivers/watchdog/wdt.c: warning: this statement may fall
through [-Wimplicit-fallthrough=]:  => 391:3

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
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>
diff --git a/drivers/watchdog/wdt.c b/drivers/watchdog/wdt.c
index 0650100..7d278b3 100644
--- a/drivers/watchdog/wdt.c
+++ b/drivers/watchdog/wdt.c
@@ -389,7 +389,7 @@ static long wdt_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 		if (wdt_set_heartbeat(new_heartbeat))
 			return -EINVAL;
 		wdt_ping();
-		/* Fall */
+		/* Fall through */
 	case WDIOC_GETTIMEOUT:
 		return put_user(heartbeat, p);
 	default: