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

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

Link: https://lore.kernel.org/r/78da5070d3ff5767da6f1c053d069c1e25229375.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>