pwm: samsung: Simplify code to determine the pwmchip's parent device

pwm: samsung: Simplify code to determine the pwmchip's parent device

There is already a pointer to the pwmchip, make use of it directly
instead of using the struct samsung_pwm_chip *our_chip just obtained
from it. This also has the advantage of not using struct
samsung_pwm_chip::chip any more which will be dropped soon.

Link: https://lore.kernel.org/r/4e8400a42979b6be0b80dff5ae964d8e9d6232eb.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>