pwm: stm32: 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 stm32_pwm *priv just obtained from it. This
also has the advantage of not using struct stm32_pwm::chip any more
which will be dropped soon.

Link: https://lore.kernel.org/r/54ace92a3c02d22f15a79c7ecf00c29f28386a33.1707900770.git.u.kleine-koenig@pengutronix.de


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