This prepares the driver for further changes that will drop struct
pwm_chip chip from struct stm32_pwm_lp. Use the pwm_chip as driver
data instead of the stm32_pwm_lp to get access to the pwm_chip in
stm32_pwm_lp_suspend() without using priv->chip.

Link: https://lore.kernel.org/r/df47d1aff9b529c9a4762b6ba339a18cecba1497.1707900770.git.u.kleine-koenig@pengutronix.deSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>