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

Link: https://lore.kernel.org/r/56d53372aacff6871df4d6c6779c9dac94592696.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>