i2c: rcar: remove sign-compare flaw

gcc rightfully says:

drivers/i2c/busses/i2c-rcar.c:198:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

drivers/i2c/busses/i2c-rcar.c

@@ -195,7 +195,7 @@ static int rcar_i2c_clock_calculate(struct rcar_i2c_priv *priv,
 	 */
 	rate = clk_get_rate(priv->clk);
 	cdf = rate / 20000000;
-	if (cdf >= 1 << cdf_width) {
+	if (cdf >= 1U << cdf_width) {
 		dev_err(dev, "Input clock %lu too high\n", rate);
 		return -EIO;
 	}