clocksource: timer-ti-dm: Check prescaler value

Invalid value silently disables use of the prescaler.
Use -1 explicitely for that purpose and error out on
invalid value.
Signed-off-by: NLadislav Michl <ladis@linux-mips.org>
Signed-off-by: NTony Lindgren <tony@atomide.com>

drivers/clocksource/timer-ti-dm.c

@@ -663,13 +663,13 @@ static int omap_dm_timer_set_prescaler(struct omap_dm_timer *timer,
 {
 	u32 l;
 
-	if (unlikely(!timer))
+	if (unlikely(!timer) || prescaler < -1 || prescaler > 7)
 		return -EINVAL;
 
 	omap_dm_timer_enable(timer);
 	l = omap_dm_timer_read_reg(timer, OMAP_TIMER_CTRL_REG);
 	l &= ~(OMAP_TIMER_CTRL_PRE | (0x07 << 2));
-	if (prescaler >= 0x00 && prescaler <= 0x07) {
+	if (prescaler >= 0) {
 		l |= OMAP_TIMER_CTRL_PRE;
 		l |= prescaler << 2;
 	}