pwm: Use a consistent name for pwm_chip pointers in the core

Most variables of type struct pwm_chip * are named "chip", there are
only three outliers called "pc". Change these three to "chip", too, for
consistency.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>