pwm: tegra: Rename variable pointing to driver private data

Status quo is that variables of type struct tegra_pwm_chip * are named
"pwm", "chip" or "pc". The two formers are all not optimal because
usually only struct pwm_device * variables are named "pwm" and "chip" is
usually used for variabled of type struct pwm_chip *.

So consistently use the same and non-conflicting name "pc".

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