pwm: jz4740: 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 jz4740_pwm_chip. To just not have to
do that, rework jz4740_pwm_can_use_chn() to take a pwm_chip.

Link: https://lore.kernel.org/r/83b826f0bf58b93f26d3bc30b1a957f84eedf41a.1707900770.git.u.kleine-koenig@pengutronix.de


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