hwmon: (aspeed-g6-pwm-tacho): 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.

Acked-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/5c6a11dd10cd29e0f7bfaa1fdef145523347cb58.1710777536.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>