pwm: dwc: Make use of devm_pwmchip_alloc() function

pwm: dwc: Make use of devm_pwmchip_alloc() function

This prepares the pwm-dwc driver to further changes of the pwm core
outlined in the commit introducing devm_pwmchip_alloc(). There is no
intended semantical change and the driver should behave as before.

Also convert the to_dwc_pwm() helper macro to a static inline to get
some type safety.

Link: https://lore.kernel.org/r/64e64a9a8644600d51b234ad9bcae951b5748998.1707900770.git.u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>