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

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

Link: https://lore.kernel.org/r/b2b06a3aabf8c04969d59ddf7ba565b303855878.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>