pwm: vt8500: 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 vt8500_chip::chip.

Link: https://lore.kernel.org/r/7d903b608609d46cf1ee1e06530f516f42af1ebb.1707900770.git.u.kleine-koenig@pengutronix.de


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