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

Link: https://lore.kernel.org/r/e588e3ccdd0ac1c9c3f3a79b3e52112f83f9d71c.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>