This prepares the driver for further changes that will drop struct
pwm_chip chip from struct lpc18xx_pwm_chip. Use the pwm_chip as driver
data instead of the lpc18xx_pwm_chip to get access to the pwm_chip in
lpc18xx_pwm_remove() without using lpc18xx_pwm->chip.

Link: https://lore.kernel.org/r/b7bf8c421ff754ec3e985ec6235f4ddd6e5e27a4.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>