watchdog: scx200_wdt: Use fallthrough pseudo-keyword

Replace the existing /* fall through */ comments and its variants with
the new pseudo-keyword macro fallthrough[1].

[1] https://www.kernel.org/doc/html/v5.7-rc7/process/deprecated.html?highlight=fallthrough#implicit-switch-case-fall-throughSigned-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20200717164059.GA26947@embeddedorSigned-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>

drivers/watchdog/scx200_wdt.c

@@ -186,7 +186,7 @@ static long scx200_wdt_ioctl(struct file *file, unsigned int cmd,
 		margin = new_margin;
 		scx200_wdt_update_margin();
 		scx200_wdt_ping();
-		/* Fall through */
+		fallthrough;
 	case WDIOC_GETTIMEOUT:
 		if (put_user(margin, p))
 			return -EFAULT;