i2c: jz4780: prevent potential division by zero

Make sure we don't OOPS in case clock-frequency is set to 0 in a DT. The
variable set here is later used as a divisor.
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-jz4780.c

@@ -770,7 +770,7 @@ static int jz4780_i2c_probe(struct platform_device *pdev)
 
 	ret = of_property_read_u32(pdev->dev.of_node, "clock-frequency",
 				   &clk_freq);
-	if (ret) {
+	if (ret || clk_freq == 0) {
 		dev_err(&pdev->dev, "clock-frequency not specified in DT");
 		goto err;
 	}