fbdev: au1200fb: Fix potential divide by zero

var->pixclock can be assigned to zero by user. Without
proper check, divide by zero would occur when invoking
macro PICOS2KHZ in au1200fb_fb_check_var.

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

drivers/video/fbdev/au1200fb.c

@@ -1040,6 +1040,9 @@ static int au1200fb_fb_check_var(struct fb_var_screeninfo *var,
 	u32 pixclock;
 	int screen_size, plane;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	plane = fbdev->plane;
 
 	/* Make sure that the mode respect all LCD controller and