pwm: mediatek: Make use of pwmchip_parent() accessor

struct pwm_chip::dev is about to change. To not have to touch this
driver in the same commit as struct pwm_chip::dev, use the accessor
function provided for exactly this purpose.

Link: https://lore.kernel.org/r/819996ee175bec0ab71acc385c78e7233617e136.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

drivers/pwm/pwm-mediatek.c

@@ -149,7 +149,7 @@ static int pwm_mediatek_config(struct pwm_chip *chip, struct pwm_device *pwm,
 
 	if (clkdiv > PWM_CLK_DIV_MAX) {
 		pwm_mediatek_clk_disable(chip, pwm);
-		dev_err(chip->dev, "period of %d ns not supported\n", period_ns);
+		dev_err(pwmchip_parent(chip), "period of %d ns not supported\n", period_ns);
 		return -EINVAL;
 	}