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

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

Link: https://lore.kernel.org/r/66d55649e13526bbd95d7bfd1cacfa0beb9efd43.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>