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>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20190729151033.GA10143@embeddedor
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/ar7_wdt.c

@@ -235,6 +235,7 @@ static long ar7_wdt_ioctl(struct file *file,
 		ar7_wdt_update_margin(new_margin);
 		ar7_wdt_kick(1);
 		spin_unlock(&wdt_lock);
+		/* Fall through */
 
 	case WDIOC_GETTIMEOUT:
 		if (put_user(margin, (int *)arg))

drivers/watchdog/pcwd.c

@@ -651,7 +651,7 @@ static long pcwd_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 			return -EINVAL;
 
 		pcwd_keepalive();
-		/* Fall */
+		/* Fall through */
 
 	case WDIOC_GETTIMEOUT:
 		return put_user(heartbeat, argp);

drivers/watchdog/sb_wdog.c

@@ -202,6 +202,7 @@ static long sbwdog_ioctl(struct file *file, unsigned int cmd,
 		timeout = time;
 		sbwdog_set(user_dog, timeout);
 		sbwdog_pet(user_dog);
+		/* Fall through */
 
 	case WDIOC_GETTIMEOUT:
 		/*

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: