pwm: stm32: Prepare removing pwm_chip from driver data

This prepares the driver for further changes that will drop struct
pwm_chip chip from struct stm32_pwm. Use the pwm_chip as driver
data instead of the stm32_pwm to get access to the pwm_chip in
stm32_pwm_suspend() without using priv->chip.

Link: https://lore.kernel.org/r/3db96cd915d9d8fc350a7193c0d55dd109b1f035.1707900770.git.u.kleine-koenig@pengutronix.de


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