fbdev: lxfb: Fix potential divide by zero

var->pixclock can be assigned to zero by user. Without proper
check, divide by zero would occur in lx_set_clock.

Error out if var->pixclock is zero.
Signed-off-by: NWei Chen <harperchen1110@gmail.com>
Signed-off-by: NHelge Deller <deller@gmx.de>

drivers/video/fbdev/geode/lxfb_core.c

@@ -235,6 +235,9 @@ static void get_modedb(struct fb_videomode **modedb, unsigned int *size)
 
 static int lxfb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
 {
+	if (!var->pixclock)
+		return -EINVAL;
+
 	if (var->xres > 1920 || var->yres > 1440)
 		return -EINVAL;