pwm: vt8500: 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 vt8500_chip. To just not have to do
that, rework vt8500_pwm_busy_wait() to take a pwm_chip.

Link: https://lore.kernel.org/r/fb384c550b359e7707219f87872bcf36482875ff.1707900770.git.u.kleine-koenig@pengutronix.de


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