pwm: stm32: Change prototype of a helper to prepare further changes

This prepares the driver for further changes that will make it harder to
determine the pwm_chip from a given stm32_pwm. To just not have to do
that, rework stm32_pwm_raw_capture() to take a pwm_chip.

Link: https://lore.kernel.org/r/33790c64563cb0434d7156d96f189c6037b3eb0b.1707900770.git.u.kleine-koenig@pengutronix.de


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