pwm: i.MX: eliminate build warning

compiling the i.MX pwm driver produces the following warning:
|drivers/pwm/pwm-imx.c: In function 'imx_pwm_probe':
|drivers/pwm/pwm-imx.c:281:7: warning: assignment discards qualifiers from pointer target type

Apply a 'const' attribute to the affected variable declaration.
Signed-off-by: NLothar Waßmann <LW@KARO-electronics.de>
Acked-by: NSascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: NThierry Reding <thierry.reding@avionic-design.de>

drivers/pwm/pwm-imx.c

@@ -235,7 +235,7 @@ static int __devinit imx_pwm_probe(struct platform_device *pdev)
 {
 	const struct of_device_id *of_id =
 			of_match_device(imx_pwm_dt_ids, &pdev->dev);
-	struct imx_pwm_data *data;
+	const struct imx_pwm_data *data;
 	struct imx_chip *imx;
 	struct resource *r;
 	int ret = 0;