pwm: iqs620a: Create a wrapper for converting a pwm_chip to driver data

There is no semantic change. The new function just implements what was
open-coded twice in the driver before.

Link: https://lore.kernel.org/r/cc7ab547b4c4b79456b548aa72205bcf4476db8c.1707900770.git.u.kleine-koenig@pengutronix.de


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