[WATCHDOG] bfin: fix max timeout calculation

Relying on overflow/wrap around isn't exact because if you wrap far
enough, you get back to "valid" values.
Reported-by: Thorsten Pohlmann <pohlmann@tetronik.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>
Cc: stable <stable@kernel.org>

drivers/watchdog/bfin_wdt.c

 /*
  * Blackfin On-Chip Watchdog Driver
- *  Supports BF53[123]/BF53[467]/BF54[2489]/BF561
  *
  * Originally based on softdog.c
- * Copyright 2006-2007 Analog Devices Inc.
+ * Copyright 2006-2010 Analog Devices Inc.
  * Copyright 2006-2007 Michele d'Amico
  * Copyright 1996 Alan Cox <alan@lxorguk.ukuu.org.uk>
  *
@@ -137,13 +136,15 @@ static int bfin_wdt_running(void)
  */
 static int bfin_wdt_set_timeout(unsigned long t)
 {
-	u32 cnt;
+	u32 cnt, max_t, sclk;
 	unsigned long flags;
 
-	stampit();
+	sclk = get_sclk();
+	max_t = -1 / sclk;
+	cnt = t * sclk;
+	stamp("maxtimeout=%us newtimeout=%lus (cnt=%#x)", max_t, t, cnt);
 
-	cnt = t * get_sclk();
-	if (cnt < get_sclk()) {
+	if (t > max_t) {
 		printk(KERN_WARNING PFX "timeout value is too large\n");
 		return -EINVAL;
 	}