pwm: ab8500: Introduce a local pwm_chip variable in .probe()

This simplifies converting the driver to pwmchip_alloc() as there is only
a single code line left that makes use of struct ab8500_pwm_chip::chip.

Link: https://lore.kernel.org/r/7fae4833b97728abc8bd400663fdd331c86a761b.1707900770.git.u.kleine-koenig@pengutronix.de


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