pwm: stm32-lp: 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_lp *priv just obtained from
it. This also has the advantage of not using struct stm32_pwm_lp::chip
any more which will be dropped soon.

Link: https://lore.kernel.org/r/9ad2399e1a683a6344b12d7f70498393b8f8b9de.1707900770.git.u.kleine-koenig@pengutronix.de


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