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

Link: https://lore.kernel.org/r/628f4b8c4ba1321075fc1dff70453a1c79ffb814.1707900770.git.u.kleine-koenig@pengutronix.de


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