rtc: m41t80: mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

Notice that in this particular case, I replaced "Fall" with a proper
"Fall through" comment.

Addresses-Coverity-ID: 1373875 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>

drivers/rtc/rtc-m41t80.c

@@ -745,7 +745,7 @@ static int wdt_ioctl(struct file *file, unsigned int cmd,
 			return -EINVAL;
 		wdt_margin = new_margin;
 		wdt_ping();
-		/* Fall */
+		/* Fall through */
 	case WDIOC_GETTIMEOUT:
 		return put_user(wdt_margin, (int __user *)arg);