pwm: rochchip: Prepare removing pwm_chip from driver data

This prepares the driver for further changes that will drop struct
pwm_chip chip from struct rockchip_pwm_chip. Use the pwm_chip as driver
data instead of the rockchip_pwm_chip to get access to the pwm_chip in
rockchip_pwm_remove() without using pc->chip.

Link: https://lore.kernel.org/r/301657442c578cc8d330ad2b07463d59fed4bff6.1707900770.git.u.kleine-koenig@pengutronix.de


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