gpu: ipu-v3: fix div_ratio type

The variable can be negative.

The problem has been detected using proposed semantic patch
scripts/coccinelle/tests/unsigned_lesser_than_zero.cocci [1].

[1]: http://permalink.gmane.org/gmane.linux.kernel/2038576

Signed-off-by: Andrzej Hajda <a.hajda@samsung.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>

drivers/gpu/ipu-v3/ipu-csi.c

@@ -202,7 +202,7 @@ static int ipu_csi_set_testgen_mclk(struct ipu_csi *csi, u32 pixel_clk,
 					u32 ipu_clk)
 {
 	u32 temp;
-	u32 div_ratio;
+	int div_ratio;
 
 	div_ratio = (ipu_clk / pixel_clk) - 1;