ARM: pxa: Make id const in pwm_probe()

This fixes the compiler warning:

	arch/arm/plat-pxa/pwm.c: In function 'pwm_probe':
	arch/arm/plat-pxa/pwm.c:179: warning: initialization discards qualifiers from pointer target type
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: NEric Miao <eric.y.miao@gmail.com>

arch/arm/plat-pxa/pwm.c

@@ -176,7 +176,7 @@ static inline void __add_pwm(struct pwm_device *pwm)
 
 static int __devinit pwm_probe(struct platform_device *pdev)
 {
-	struct platform_device_id *id = platform_get_device_id(pdev);
+	const struct platform_device_id *id = platform_get_device_id(pdev);
 	struct pwm_device *pwm, *secondary = NULL;
 	struct resource *r;
 	int ret = 0;